I did a non-exhaustive search through past forum posts and could not find this answered:
Can a character have both one of the Faerie Blood virtues and Mythic Blood?
It doesn't seem explicitly disallowed? (Though I could be missing something.) I did see one mention of an SG allowing it but that is far from conclusive.
A ridiculously powerful merinita mage having a relationship with a Faerie. Seems logical.
My general thought is if the rulebook doesn't specifically disallow something, it doesn't go against any specific house rule the SG has put in place previously, and a player wants to do it, let it happen.
They're mechanically compatible and neither necessarily needs to be a close biological relationship so the fluff's OK too though I would wonder whether they're both necessarily needed for the player's concept as they are kinda similar thematically when you get right down to it.
In my specific context I was thinking Faerie God Blood (Appollon) and Hyperborean Descent (The Hermetic Major one). This maybe adds a bigger wrinkle around do the aging modifiers stack?
General theme I've seen around stacking is No, highest applies.
Stacking creates lots of rorts. Such as a low level spell that adds +1 soak, so just spend 15 minutes before you head out, +50 soak later, and you are nigh invulnerable with a spell that's not even level 10.
I could see the argument in this case that there is no stacking but generally, in the case of virtues, they do stack.
Hi,
Virtues almost always stack. Spells that do the same thing in the same way do not stack. A spell that provides +1 soak by making your skin tougher does not stack with casting it again, or with some other spell that makes your skin tougher, but does stack quite nicely with another spell that grants +1 soak by dressing you in leather armor.
I'd look at what the spell or effect is doing. Does one Imaginem spell improve your situation by giving +3 to social rolls because of your sweet voice? And then you have another Imaginem spell to make you look handsome? And another to make your clothes look like the right fashion for your appearance at court? These ought to stack.
Anyway,
Ken
And, yeah, one of the things that came to mind when I first thought of this was the son of Emrys who was apprenticed to Quendalon and began the Heirs to Merlin lineage within House Merinita. He most definitely had Faerie Blood and probably had Mythic Blood (Emrys?). Mentioned on p99 of HoH:MC.
I don't see why not?
That is a topic which your troupe should decide.
I don't see, why such a character would violate any rules. Both Virtues even might synergize nicely. But I imagine, that a few sagas would not allow it because "too strange".
Earlier I let a player to have such virtues and flows, which resulted in a total of -11 penalty on Social rolls among mundanes. (It was not a wise idea...) So that character was not really playable outside of the covenant.
I feel here the same could happen: the Gift (-3) plus some modifier due to the Faerie blood (according to its strangeness) and also there could be due to the Mythic Blood. So, that character would have a big disadvantage among humans (similar like to Blatant Gift).
Your troupe should decide whether to accept it or not.
I don't think there is anything specifically disallowing that combination. At the same time there is the statement "You may not have both Faerie Blood and Strong Faerie Blood" (ArM5 p.49). Why could you not have dwarf blood from one parent and sidhe blood from another, with one of them being stronger, for instance? Ultimately, I don't think there is anything game-breaking at all about it, not even with stacking those aging bonuses, so if the troupe is good with it, why not?
Now THAT is a blatant gift worthy of that name xD In fact my last table troupe went for -5 for normal Gift and -10 for Blatant Gift, so your numbers do not seem that weird to me. It added a lot of value to the Gentle Gift and mundane companions and grogs, and a lot of creepiness and thrown cabbages at the mages. I liked it once I managed to get my stride with the change and not stumble with it regularly.
I would let the combo fly if it was not made from a pure powerplayer approach,. And even in the later case I would allow it if the saga is OK with powerplaying. A faerie princess is attracted to the mythical descendant of a dragon because he is the mythical descendant of a dragon and not just a plain old boring vanilla ruler of France or Constantinople. And BAM! A double strong blooded dude or gal. Easy 
Cheers
Xavi
Hi,
Not playable outside of the covenant? Just two words: Rego Mentem. 
Anyway,
Ken
Or MuMe(Co) to manifest a person's mind as a human.
According to the rules, there doesn't seem to be any reason why you couldn't create a mage character with the virtues Mythic Blood, Faerie Blood (the minor virtue)), Giant Blood, Guardian Angel, and the flaw Diabolic Past or Plagued by Supernatural Creature (major demon).
I doubt that I would want to play such a mongrel character.
It could also be mentioned that not every 'faerie blood' character traces their lineage to a faerie. You could easily be 'faerie blooded' by just being kidnapped by faeries as a child, and carrying some of their magic back with you.