Well, you see, hydra heads don't keep a full set of organs in each. Those things would be like fully equipped humanoids, two of them, but as one, with the only connector being the fleshy tube with bones in it. Aside from that, bilateral symmetry, two identical halves.
So, I doubt the game would try to parse their splitting into an expected result of creating two living split halves, each counting as a creature, so... which half would become a cut off bit?
First off, shame on you for posting off topic.
To answer your question, though, DF doesn't consider anything other than the body tree when determining what happens when a part gets severed.
In other words, the fact that both bodies have full sets of organs etc is completely beside the point. The only thing that matters is how the body parts are connected to each other.
Therefore, you're question actually can't be answered, since it depends on how you set up the BODY plan; but a good guess is that whichever body was mentioned second in the raws would be the one that gets severed.
More importantly, however, the creature would probably die instantly, because creatures die if they get a HEAD, LOWERBODY, or UPPERBODY cut off, even if they have spares!
I'm still not 100% clear on this, but I'm breaking it down step by step as it seems:
1) Two castes were made for a race: one with a quadruped body, and one with a humanoid body
2) The castes bred, resulting in a baby with a quadruped body AND a humanoid body
Is this really possible/intentional? None of the posts in the thread have made it entirely clear.
NOTE: I see you understand now, but I'm leaving this in anyway
. Darn Ninja...
It is not possible. 99% sure.
I think we're all pretty sure that point 1) is not true, ie, we think the original poster made some kind of mistake when defining his castes, either he had more than two castes or the quadruped caste was actually a quadruped/humanoid, and he only noticed on the baby.
I agree with Deon that there probably isn't a bug. The rules for multiple BODY tags are different than the rules for multiple tags of other types, but this is probably WAI* from Toady's point of view.
*Working As Intended