WEasy answer. The caster of the spell that ends up affecting the foreign aegis (Tellus, in this case) needs to penetrate the other aegis. The other aegis does not need to penetrate since Dophus is just a dupe with no MR.
Xavi
True, but unfortunately this implies nothing. For example, dogs have four legs. I didn't mention anything about cats. Does that mean cat's don't have four legs? That the big debate on the Berklist (I think it was there.) way back when was about wards is why the official statement was about wards. Essentially that's like asking me how many legs dogs have being followed by my reply above that dogs have four legs.
Chris
No, no, not irrelevant at all! Parma Magica can now (with penetration used) allow magi to approach covenants with their familiars. What happens to some of my characters in non-penetrating games is that they won't walk up to (figuratively) another covenant because they won't just leave their familiars behind. Also, you need to have two tokens per visiting magus, not one.
How is this any different from a circular ward? Try to teleport across a circular ward. What is it penetrating against? Whatever ruling you use for the circle should be what you use for the Aegis.
Chris
Let's see: spells cast from outside the Aegis must beat the Aegis level with their Penetration. The wording is even "if the spell cannot penetrate a resistance equal to the level of the Aegis". That's a very Magic Resistance-like behavior. In fact, it is exactly Magic Resistance's mechanism.
Creatures with a Might cannot enter unless they have a higher Might than the Aegis level. Since the Creature's Might is its Penetration, once again this is very Magic Resistance-like.
Uninvited magi casting inside the Aegis subtract half its level from their Casting Total. Okay, that is a different mechanism, but then you cannot cast from within someone else's Magic Resistance.
No, to me the Aegis is best described as a form of Magic Resistance that affects an area.
Besides, it is supposed to be a very powerful spell. "more powerful than it ought to be" reads the description. It netted its inventor the title of Primus of House Bonisagus. To me, that means that magi with a good Vim Form resistance are not supposed to just waltz in and ignore it. Neither should it be easy for magic creatures to do so. Getting invited is a big deal even for an hermetic magus, if only so he familiar can come too.
Of course they sound very similar when you leave out most the differences while comparing and contrasting them. How about these? Aegis can block Intellego spells Parma Magica cannot block. You can cast inward through it if you were involved, but you have to beat your own Parma Magica to cast inward through it. Your Parma Magica resists all your own devices except your talisman, while all items from within the Aegis are fine. The whole tokens thing.
Even if all it did was hold creatures at bay, that would make it drastically more powerful than all the same level circular wards, qualifying for that statement. So that statement doesn't help at all here.
You seem to have misread things. Getting invited in does not allow your familiar to enter. Your familiar needs to be invited separately and needs its own token.
Chris
Spells and supernatural abilities don't generally have MR so most of Aegis's effects don't need to penetrate. So the major concern is really just aegis's ability to exclude creatures with a might score. This makes your ideal penetration total equal to the Aegis's level just like wards.
I did the math and wizards communion really helps with penetration totals it's surprising. With an Aegis specialist and even a modest sized casting group high level Aegises with equivalent penetrations totals aren't that hard. If he can reliably hit say a 60 (for the sake of argument and easy math) then with one other Magus to help him he can put up a Aegis of 40 with a penetration of 40. 2 helpers it's 45. 3 helpers it's 48. 4 helpers is 50. So on and so forth and this is without figuring in the penetration ability or a mastery with a penetration specialization. Of course with small casting groups the wizards communions might be prohibitively high but once your dealing with a casting group of around 4 or 5 the communions only need to average in the twenties.
Assuming both magus know Wizard's Communion at level 40, or some other repartition. With 2 helpers, 40+20+20 is easier to reach.
I have to echo this arguement entirely. We play that an Aegis needs to penetrate
I think this makes sense otherwise when a Magus walks into a foreign Aegis, then his Parma as a magical effect would also be resisted (as per wind of mundance silence), as many Tribunals are likey to be protected by an aegis, the younger magi, walk in, loose their parma and feel the full force of the gift. Accordingly Chaos ensues.
Again a Ward of Lvl 10 won't stop a lvl 15 beastie, even if it has a penetration of 100. As I see an Aegis as no different to a ward in this regard then An Aegis of level 25 won't stop a beastie of level 30 even if it has a high penetration. But I still see no reason which excludes the penetration.
Wizards communion becomes INCREDIBLY useful in this situation. Having 1 Maga per magnitude cast the spell means the level drops accordingly and the penetration skyrockets. If the main caster has a total + penetration of 30 all together then on wizards communion with 5 magi on a level 25 aegis means the spell goes form penetration 5 to penetration 25. There is flavour text about how major council meetings and feasts follow the casting. What could be more symbolic then the magi comming together the cast the spell that holds them all safe.
The only thing where I think it could be argued it DOESN'T need to penetrate (and I can be swayed either way here) is the casting total alteration. Is the spell suppressing the spell as cast, the ambient (fluid) vis or directly effecting the Magi. If it's the 1st 2 then no it won't need to penetrate, if its the 3rd then yes it would.
A
But what about the mastery "magic resistance" when it comes to aegis?
If the aegis has to penetrate, doesn't that mean that every battle magus will learn an aegis (level 20 or more), and spend 1 season to master it in that way, so that their effective MR is doubled against foreign aegis?
And, if that is the case, they can walk in enemy aegis during wizard's war, and vice versa. (According the fact that a parma of 4 (in vim), + a score of 5 mean 60 magic resistance, and is not hard to have... but having an aegis with 60 penetration is not that easy. (Let say a level aegis 40, that needs 8 magi in the communion and a initial casting total of 60+(40/8)= 65. Which need, let say 18 in vim, 18 in rego, 9 in stamina/artes/philo and a roll of 5, and penetration ability 5)
What effect does the Aegis have on the magus, that you'd want extra MR? They don't have MM and, except for the few personal spells they will be casting within the Aegis, there's nothing left to protect.
I don't want to live in a world were Aegis has to penetrate. 
Besides the fact that the idea of wards having to penetrate instead of the acting party having to beat the ward makes little to no sense to me outside of the game, the entire purpose of a Aegis is blown if you have to stack 60-100 levels of penetration on top of it.
Axiom is: personal spells are under the parma, since parma can't stop them to affect you.
Hypothesis: you are invisible in an enemy aegis, because the aegis has not penetrated you due to your aegis mastery magic resistance.
What happens when you go into magus A sanctum during the night, seize a dagger [which doesn't become invisible since it was not part of the initial spell, hence is not magically invisible and doesn't bounce off his parma] and kill him?
During a wizard's war, this is really bothering.
In my opinion, the aegis not penetrating prevented that trick: your magus would become visible and grogs - who are supposed to guard the sleeping magi - can see him.
The magus didn't use any spell inside covenant, but the fact that the aegis couldn't dispell his personal invisibility when he entered totally ruined its purpose.
Well, when those 'personal' spells start with a Ball of Abysmal Flames and follow up with a Clenching Grasp of the Crushed Heart, you really want an Aegis to do something. Because, you know, an Aegis is supposed to provide some defense during a Wizard War...
The point is those spells don't have MR so they would be effected by the aegis no matter what the penetration was.
In an aegis must penetrate, a hermetic breakthrough idea: give MR to spells.
Which give me good ideas of stories huhu.
Test: 4 magi use WC for an Aegis 40, rolling 62 as casting total. Penetration is 52. Attacker's Veil of Invisibility will fizzle if his casting total was less than (20 + 40/2 = 40), but only if his Vim MR is less than (52 / 2 = 26 with mastery).
Test: a magus cast Aegis 40, no penetration. Attacker's Veil of Invisibility will fizzle if his casting total was less than (20 + 40/2 = 40). {Much easier as you could roll 32 and still get it up, but I wanted the same numbers.}
So, your problem with penetration is that it is much easier to get MR 26 than to get 60+ in a ritual?
I don't understand at all what you wrote 
The aegis effect is:
1- prevent might creature to enter (as a ward would)
2- anyone inside which was not invited or haven't cast the aegis suffer level/2 malus in casting total.
3- block and dispell foreign spell even if it couldn't be blocked.
4- item must beat a MR of the aegis level
So with the debate: two possibilities.
- if no penetration needed:
1-> compare might to aegis. If might >, the creature enters without trouble
2-> no problem. You are invited or not, you have cast the aegis or not.
3->no problem. Any spell of an uninvited magus or not present at the time of casting is blocked
4- no problem. Is penetration of item enough or not to beat aegis level?
If penetration is needed:
1-> does the aegis penetrate if the might < level of aegis? No change for might> aegis.
2-> If you are not invited and weren't present at the time of casting, this effect only apply if: you are magic creature or magus casting a non personal spell. (since spelL/power has no MR)
3->if the aegis can't beat your MR, any personal spell on you is unaffected, even if you are not invited and weren't there at the time of thea egis casting. (here is my example lying)
4- no problem. Item has no MR so it's the same.
My problem lies in 3.
And yes, I think it's way easier to have a big specific MR against aegis than to have a big penetration against specific individuals.
So here is a question? Does Parma have to penetrate? It is a magical thing created and sustained by magic right. What ever sort of ability generates Parma, it still comes crashing down under a Wind of Mundane Silence. That sounds like sustained by magic to me, and I don't think there is any specific exception written for Parma not needing to penetrate.
So Loganus the combat twink buffs himself up with super sharp diamond claws coming out of his knuckles using a personal spell and stabs you in the heart. Congratulations he doesn't have to worry about penetrating your Parma, because all that magic resistance is magically created not natural. Personal spells protected by Parma, awesome.
Wait but your Parma is also magical so they cancel each other out right? Nope if we are going there Parma is in fact protected by itself (again description of Wind of mundane silence as the example) So the bigger Parma always wins. Parma specialist stealth assassin here I come.
So a Magus, all his personal spells, all his cloths and any items literally "very close to him" (so enchant the hell out of those things) are unaffected by a Parma Magica lower than his own, no penetration necessary. On the plus side form resistance is apparently natural so there is that.
Parma is not a supernatural ability but an arcane one.
And if your post is to say that the aegis having to penetrate is absurd like would be a parma needing to penetrate another parma, yeah, i agree.
in fact, your whole post sound like a joke so I will just ignore it...