Here's a short summary of my current "research" on this "topic":
The human half mainly serves as a "strange neck", with no organs, aside from a heart and a small lung for fine control of airflow (i.e: speech.), and is also connected to the horse lungs, similarly to avian respiratory systems.
Location: 2 pair of small ones in the upper (human) ribcage, 2 pairs of large ones in the horse part.
Btw, meeting the oxygen requirements of a running horse is easy, as they can't breathe through their mouth (another nail in "intelligent design's" coffin), but humans can. For further information visit the following link: https://youtu.be/QsvS6gEBJuE?t=1497 (Warning: Autopsy)
The digestive system is redesigned to be omnivore, with the addition of being able to digest grass.
Location: Solely in the horse-half, aside from the esophagus and the mouth, of course.
Circulation, is pretty much intact, aside from a few rewirings but with the addition of a human small heart, that helps at getting the blood up to the brain.
Did I mention you the real Time Lords? (yes, there was a DOUBLE HEART ATTACKT, no, he survived it.
Stronger bones in the front legs, plus a lighter human-half would be ideal.
The human-half shouldn't contain much fat, the horse part seems to be more fit for that.
And for everything else, there's the Internet and Master Card.