I would indeed not think so, as these would be stacking Muto spells.
Here I see assumption of a different Size and transformation into wolf interacting and overlaying their effects. If the result shall include both effects, it should be achieved by a single one of the Muto spells.
@Samiloth: You basically beat me to it and provide good reasoning. Looks like I am the slow poster here again and again. 
No, that is not exactly true. The rules state Corpus effects can still affect affect the shape-shifted person.
That actually directly contradicts the rules. It might not win out, but it is quite explicitly still active rather than canceled.
Its effects may be overwritten, but that is so according to canon. This is what I'm trying to understand in your interpretation because I think I like yours the most. But I have to figure out how to connect this with such effects continuing, as it is stated they do in canon. They may continue while being overwritten, but they do remain active and have their effects.
OK. I wasn't sure. Why I wasn't sure is if you use increase Size, your eyes increase in size as well. But they're under their own Muto effect to be cats' eyes. So I was trying to figure out how you mark the cut-off between overlaying and non-conflicting.
In this case I would accept a Mu Co size change spell, or a straight Mu An. Because I read the wording for shape change as “ either or”, because it is a weakness of shape changing
If you change a fat, one eyed man into a wolf, you get a fat one eyed male wolf.
If the human changed is large (size +1) I assume you get a larger wolf.
As per Ringing the Changes HoH: MC “any ongoing magics using these Forms (referring time Corpus and Mentem) cast prior to transformation remain in effect”
So a Corpus size change spell before the wolf spell should result in a larger wolf. Because the shape changed human is affected by both Co and An the size change remains in effect even after change to wolf.
If the change to wolf comes first, it is still affected by both Co and An, so any of these forms will change size.
Could you please tell me, what you mean by a spell with 'overwritten effects? A spell without a current effect, but which might provide an effect once another spell no longer is effective?
And where is your quote for canon?
The problems with stacking Muto spells are indeed with their effects, and not with their magic and durations. But being precise here requires redefining a few terms like effect (which can be a spell effect or an effect in a device) and spell (which is often the effect of the magic in a spell). That work exceeds the time I can allocate here at the moment.
I recall that the Bjornaer part of HoH states something about using corpus for animal if someone is transformed. It makes no sense and is a good example of something that isn't thought through. It basically means that you can turn a person to gold, then use Corpus to affect the gold, using the Corpus guidelines for Individual instead of Terram. If you turn Aquam into gold then you use Aquam and it's guidelines. Roflol?
Imagine how you can sidestep the guidelines that balance each form. Come on, it makes no sense and must be ignored. Throw away the HoH book if you need to. The effects of such rules are enormous and hermetic magi would use it to their advantage.
I did not know that what I said about size directly contradicts the rules. How did they reason in the rules I either missed or forgot?
The more I think about the Terram and Aquam examples the less I like Ringing the Changes. If may be because “other shapechange needs to be weaker than Heartbeast “
But the insert box only talks about Corpus and Animal, so the examples of Terram and Aquam aren’t addressed . IMHO the dual Forms only applies for human to animal shape change.
I have no explanation as to why this should be - except it applies to two living, sentient forms and not to the inanimate ones - but I can accept it.
Then you realize the rule as written makes no sense and ignore it by stating it only is for the forms Corpus and Animal. I think that is perfectly fine. If you had an explanation to why then I would think it was great.
I have played games without dice or other elements of chance, I have played games without rules, I have played games where everyone is GM at the same time or none is. It all works if players want it and know how to make it great. I still think rules are important, though, when games actually have them.
Here is the things about rules in a RPG: It is the natural laws of that world. If the rules say people get weightless by drinking wine, then you get weightless when drinking wine. Since it is the same to all people the world itself would be formed around that fact. No-one would for example drink wine out in the open in fear of floating away.
What kind of world people want to GM or play in differs, so people should change rules if they don't like the natural laws of that world. What exactly the rules are isn't all that important as long as it is consistent, makes sense and creates a world GM and players like. My players do for example like social roleplaying so the Gift isn't as nasty as in the vanilla rules and it isn't a major virtue to have the Gentle Gift.
My players also want the world to be a dangerous place, a place where there are risks that might end them, so we nerfed the Parma (it was either that or remove it all together). They want that if they piss of a powerful god then that god should be able to come down from whatever eternal realm it is from and give them a mighty curse, a curse that can affect not only them and all they love, but all those they will love in the future will be instantly affected as well. Then they will learn to live with the curse or find a solution, like years and years of lab work or more likely adventures to try and earn the forgiveness of the god, perhaps even finding a way to defeat it etc.
I don't think I can fully explain my dislike of the Parma. I should end all my post by stating my dislike. I have read posts here that suggest that it would be made even more powerful. It boggles my mind but if people want it, go for it. It's like playing DC superheroes from the Justice League in robes in a medieval setting. That can be cool.
So when people writes a game and have rules in it, then those rules explain the natural laws, how the world function. It gives us a view of how the game's creators imagine the world to be, how they want it to work. You can for example conclude that they don't really want players to have meaningful social interaction with the normal society because of the Gift and how costly it is to get the Gentle Gift. Magi should probably be out battling supernatural threats because they are pretty much immune from harm, not chatting with some mere mortals.
Change the rules or don't change them, both is fine, but none should take them as gospel and preach about what the rules say if they don't even accept the implication of the rules, what the rules would naturally lead to. The rules build the world, then we play in it.
Parma delenda est
By overwritten I mean like the prior original post's question. If you MuCo(An) into a wolf for Moon and then MuCo(An) into a bear for Sun, you can only really be in one of the two at a time. I think most of us would have you in bear form. If the bear spell has more penetration, is more powerful, and is cast later, most people (based on past discussions of conflicting magic) would seem to have the bear effect take place. But once Sun is over, you would return to wolf form because the spell is still ongoing. That's the kind of thing I mean by "overwriting."
I ran out of time, but Christian posted it:
.
.
.
If you look at various example spells, like turning fire into a gem in the core book or turning air into stone in some book, or even growing something really large, this isn't much of a concern. Both sizes are taken into consideration, and if you want something more on either end, you'll need to add magnitudes. So I really don't see the exploit we're worrying about here. Sure, you may in some cases be able to work with a slightly different magnitude by using a few spells, but you could already do that with MuVi plus an original so it doesn't seem out of line.
You can use other Techniques than Muto, for example Rego, right? If you want to affect a man turn into a wolf, is it only Muto Corpus you can use, not for example Rego or Perdo? If you want to control a iron ball made from a pond, then you could, with those rules, use Rego Aquam with a base individual of a pond.
I am not saying that it is wrong to allow it in your game, I say that I can't see the good of it (really, it makes no sense to me) and the consequences such rules will bring to the world. If people have no problem with it, then go for it. I have played weirder games then that TBH.
Parma delenda est
I think it is partly for consistency with other parts of the core rules. For example, what Form do you use to move a wooden staff? It's solid. It's wood. So you can move it with ReHe, or with ReTe plus a casting requisite of He.
I believe in Christian's theory. They added the rule to make Bjornaer magi special.
Parma delenda est
Two more things have been on my mind from this:
I wonder about shape-shifting into gold. Ignore the whole movement issue, maybe it's more of a Medusa thing. I'm just thinking about the size of Individual. It seems like all such spells pay attention to Individual on both ends, both before and after transformation, as we see large Individual stuff (like Auram) producing much smaller Individuals of other things and as we see significant growth requiring extra magnitudes to handle the even bigger end state. Turning someone into stone isn't an issue as a cubic pace is still large. But for precious metals, a tenth of a cubic foot is pretty small compared to a person. Would we need to add +2 size to turn a person into a golden statue of a person? I would think so.
Whatever exactly we consider stacking of magic, we should take CrFo into account for creating something that may then be modified. In canon we can CrIg fire into existence and then MuIg that fire. This does not necessarily imply much of anything about different takes on stacking, as it could instead imply something about the existence of what was created. Maybe the created fire has an essential nature of fire for its finite existence. I just thought this MuFo on top of CrFo would be good to keep in mind for cases like casting The Wizard's Mount and following it with Steed of Vengeance.
Yes, that is my reading of RAW. Gold is more noble than stone, so it is harder to produce (one way or another, even temporarily or unnaturally) a man-sized mass of it compared to stone.