You know, cows have four stomachs or stomach-like organs, and animals in general have redundancy in their various systems... Maybe for a creature like a centaur, some organs need to have redundancies for maximum efficiency? I can definitely see the use of two or more hearts to properly circulate all that blood. And maybe a couple of stomaches to handle the differently sourced nutrients needed for a horse body and human body/mind.

I won't even touch the bone structure nightmare. I'm no fucking mythical creature doctor.