tldr; you need to stop thinking of this as a human + horse and think of it as a totally unique animal that is neither.
There is however one obvious problem
...No, there are many, many problems.
- Ribcages. A ribcage exists to house and protects vital organs and regulate thoracic pressure. How and why would an animal exist with two? It wouldn't. Even if somehow responsibility was shared between organs in both thoraces, there's absolutely no way to fix the problem of...
- Oxygenation and circulation. The horses lungs are useless. The only way to move oxygenated blood to the horse extremities is by totally redoing blood flow started in the human torso. You would need much more powerful (and sizeable) lungs and blood vessels, i.e., a much bigger human torso and much smaller horse torso (since we've removed the lungs). So now we need to rethink...
- Muscles and skeleton. Since you've now totally changes the weight distribution on the skeleton, all of that is up in the air as well. The front legs would need to be much stronger, which means thick bones... much thicker than even a workhorse's front legs and certainly much thicker than your femur. With such an odd gait, a centaur would be much worse at short-distance running than a horse and much worse at long-distance running than a human.
To me, these are the most glaringly obvious problems, not to mention all the other systems in the body that wouldn't make sense.
There is a reason this animal doesn't exist in nature: it does not make sense. Instead of trying to merge a human and a horse, I would suggest creating a new animal that could exist and might pass as a centaur.
In the same way as a rhino might pass as a unicorn ;)