A sexy idea, but one that sounds like a nightmare to code.
Eh, in my opinion it's not much more of a nightmare than the centaur problem in general (of combining them to begin with); the extra feature of separating the creatures again actually seems
relatively easy once you figure out how to combine them, and Toady already wants to/plans to solve combining them (ie, he doesn't want to manually write centaur or griffon raw files/body plans, he wants to generate them by combining human and horse, and lion and eagle).
Sure, it also has its own challenges, but these all seem to need solving elsewhere too (for creatures with multiple souls in general, werebeasts, and shapeshifters).
The chimera would have to keep track of which body parts came from which origin creature, so that any injured body parts leave with the correct donor.
Tracking this shouldn't be too hard; producing the combined chimera to begin with (ie, solving the centaur problem) will probably necessitate creating the individual creatures first (in the generating code, not necessarily in the lore, since the mythgen might say the "combined" version came first), and so it would be possible to record which parts came from which creature, and indeed even which parts are
combined from each creature (eg, maybe it only has 1 heart, but its size is taken from one of them, and immunity to acidic blood from the other).
In that same vein, would an attack that strikes a fatal blow to the vital organs of one part of the chimera always result in the death of that donor, or would it possible for the "dead" donor to rely upon the systems of its merged fellows, and perhaps heal enough to survive on its own?
I'm not 100% sure what you mean here; bodily speaking they would be one creature (though with tracking of each body part's origins, but this wouldn't affect the actual mechanics), so if you destroy the heart, both will die, and if it has 2 hearts and you destroy one, both would live (provided 1 heart is able to sustain the whole body).
The only exception I can think of here is the brain/thought center, is that what you mean? This is mostly a question I think of how souls/minds are handled, and whether the soul can remain in the body if the head is destroyed but the body remains alive, or if it can return when the head is restored.
While I'm not sure about this (so I've asked in the FotF thread to find out), it's possible Toady may be planning for ettins and hydras to have a soul/mind for each head, and I know for a fact he's said he has a currently unused "multiple souls per body" system for some future purpose or another, so I think he may need to solve this issue regardless; if you destroy the brain of one ettin head, and you use a healing spell (as opposed to a proper resurrection spell), can the original ettin come back? Will hydra's regrowing heads each have a new personality every time, or the same personality but with no memories, or just be the same?
So yeah, chances are this problem needs to be solved regardless of this suggestion. It could plausibly go either way frankly, or may even vary from world to world (or even creature to creature) depending on mythgen.
I see that Kragos + Kildor has only 2 arms & 2 legs--did the others vanish upon merging? If so, where did they go? If K&K is slain while merged, will the missing limbs reappear? If Kragos, just before he merged with Kildor, was holding something in the hand that he knew was going to disappear with the merge, then would it still be in that hand once they separated? What if that arm was injured--would the injury remain, exactly as it was at the moment of merging, or would it have healed with the passage of time?
The others pretty much vanish I suppose. Or, maybe they stay "metaphysically tied" to the arm, so each arm "represents" 2 arms (even if maybe the bodily mass of the arms themselves goes elsewhere to make the creature as a whole larger); would be the same as a mad wizard merging a man and a horse, or a lion, snake and goat, or just shapeshifting magic in general (like, if you make yourself some wings and then make them go away, what happens to the wings? Shapeshifter baggage is tricky to solve).
If slain while merged, I'd assume it would depend on the creature; there are only 2 ways it could really go, the body would almost definitely stay in one piece if their true form was the merged one, but if they were naturally separate and just using magic it would depend on the mythgenned "metaphysics" of the magic involved; perhaps the magic makes a lasting change that must actively/willfully undone (so their bodies stay merged), or maybe it has to be sustained by the casters will or "their life force" and so they just turn back to their true separate forms when they stop sustaining the magic (maybe they were cursed, and all curses undo themselves on death in this world?).
As for held items, technically Kragos and Kildor couldn't hold anything because they only have hooves (the fact that their chimera form is humanoid is a bit of artistic license I guess), but to answer your question, I'd guess either the object would just fall out of he hand as it merges into the body, or it would just stay in one of the remaining hands if that body part exists on the new body; for example, Kragos holds a goblet in his left hand, Kildor holds a goblet in his right, and when they merge, they have a goblet in each hand, even if technically both "merged arms" appearance are derived from Kragos. Of course, if both have the same hand full, one of the objects
would just have to fall to the ground/be dropped.
As for arm injury, Toady has mentioned something about changing werebeasts to potentially sustain wounds between forms (so if you injure the beast form, the human/dwarf has a corresponding injury), so a similar problem arises there, and for normal shapeshifters.
Maybe the injury would somehow "carry over" to the merged form in whatever way makes sense, and heal there when they split again (because as mentioned above, the arm may be "magically associated" with the matching arms).
If one donor of a chimera has a syndrome, is it guaranteed to spread to the others as well? What about vampirism? What about lycanthropy? Can one donor change with the full moon, while the others do not? If so, might the were-creature attack its merged fellows?
This I think is also part of "the centaur problem"/merging, so it's not really a mark against this suggestion specifically (which is just about
splitting); it will depend on however Toady decides to solve this problem for merging creatures. To make a guess, I'd imagine it would probably depend on how the syndrome spreads, as if it spreads through the blood, then yeah, probably would affect both, but if it's just a curse on them as a person that cannot spread, probably not.
The question of whether an individual can shapeshift in general while merged though is an interesting one, and I'd guess yes (because Toady, the madman, has mentioned wanting to solve things like "turning someone's hand into a spider", so transforming specific body parts on the chimera probably will happen). I'd guess the werebeast half would attack the other (well, if hypothetically the syndrome couldn't spread), but then the other half would quickly split off to more easily defend itself.
A far simpler kludge, that's almost as sexy, would be to have the occasional Forgotten Beast actually be multiple beings that simply act as one. For instance, a "monstrous humanoid covered in hungry heads! Beware its projectile mouths!" Its primary means of (ranged) attack is to throw its heads at people, the heads can move & attack on their own, and if not in combat they will try to return to their host body.
Funny enough, I was writing a suggestion along these lines just now actually! (not finished yet though)