If the horse heart fails, the human heart might just be strong enough to keep the rest of the body alive as long as you don’t move much. Probably not a long term solution, but it’s better than nothing.
I am unsure whether centaurs have a joined or separate circulatory systems. Worst case - they're separate for horse & human body.
But if they're joined, then I think a centaur may survive on 1 heart, as you've said.
Yeah... honestly, it depends on how they're built. Because it might work on one of interpretation of the centaur's internal biology, but not on another.
Still funny though.