"Centaurs have two rib cages"
"Centaurs have two digestive tracts"
The human torso is simply the neck of the horse. Now this may seem a bit weird, but this is a fantasy creature so hear me out. Anatomy, especially with how it can adapt, is incredible so I propose that the spine would more towards the middle of the human torso as seen in Figure 1 which is where the cervical spine of the horse would be (Figure 2). Along with that, the human torso wouldn't be as long as most people draw them, and it would be closer to the length of a horse (average length of a male human torso is 43-45in/109-114cm, not including the head/neck which can add an extra 11.9-13.7in/30.2-34.8cm versus a horse's neck which is approximately 36in/91cm. Please keep in mind these are rough approximations from a few google searches). Here's where things get more funky, instead of an extra ribcage to keep that human shape, the horse neck muscles, and some human torso muscles, would adapt to the shape of a torso while still keeping some of the aspects of the human musculoskeletal system. I haven't put as much thought into the the chest structure itself, but since the clavicle would remain, I would assume so would the sternum.. ish. The main thing that I focused on is the muscles. Figure 1 shows the approximate layout of these muscles with the human face and arm muscles remaining the same along with keeping the major muscles of the horse's neck. However some muscles from both human and horse, although more of the horse's anatomy will need to change to become "flatter" (more human torso like). I did not add the neck tendons/ligaments to Figure 1, but those would also change configuration to comply with the form of the torso. The function of the human torso would act in place of the horse's neck with a modified horse (maybe mixed with human) digestive tract (which I will likely dive more into at some point because it would likely need to change in order to accommodate the more human style diet I have no clue, but the digestive tract would definitely need to change since the human teeth are not suited for how a horse grazes which messes up everything in the already awful thing that is the horse digestive tract)
Figure 1: Proposed centaur musculoskeletal system
Figure 3: Horse muscle chains
Figure 4: Horse superficial muscles
Anyways this was just done for fun, if you don't agree, that's fine, if have different ideas I'd love to hear them! I fell down this rabbit hole yesterday after getting into a conversation about centaur anatomy and hating the two digestive tract/rib cage ideas so here you go, the idea of the horse neck = human torso Centaur is released into the world so y'all are free to do with it as you will
Me, just trust me bro (all jokes aside, I'm a pre-vet biology major and am working on becoming certified in equine massage along with being a huge fan of fantasy creatures)
Horses Inside Out: Gillian Higgins
Human: not sure of original source, found the used image on Pinterest by a few different people
*none of this can be proven as a fact so it is not a definite "wrong" I just needed an attention grabber