Will your skeletons share some animations? if the answer is no, and you say they are similar but they're not the same, then maybe it's best to have a different skeleton for each of them.

If they have the same structure, without bones of different proportions, but with different animations, skins would be a good choice, and then you'd just make the animations keeping in mind which character should have that animation.

If they have the same animations but not the skeleton, you could either clone the project file and try to readapt the bones to the new structure (this is a painful route though, you'd probably still end up redoing half of the animations and spend time fixing things that the proportions messed up) or if they're really very similar and simple, you can attempt what was suggested here: can spine 'remember' image compensation per skin