I'd argue this is different from a centaur. Since horses have four legs and humans have two, the womanoman's graft would need to be towards the posterior of the forward woman, instead of above the hips like where the upper part of a centaur's graft would be. Two sets of human legs indicates there should probably be two sets of.. erm... equipment.
That's a good question... I think for this to work properly, the digestive tract can't just stop in the middle, they've got to be plumbed together into series. That means the first one would have to be disconnected, so if it was left in, it wouldn't be functional in its typical evolutionary context.