I'm still new to Spine myself and thus can't say for sure if it's possible to copy animations between characters/skeletons. But from what I understand, animations can be reused if the skeletons are equal in both the number of bones, the hierarchy as well as the bones' names. I've imported animations (from the main menu => Import project) from different older revisions of a test project I've been working on into one big project file. The skeleton was the same in all these files and importing single animations worked without problems. Maybe you can do it this way, too, assuming skeleton A and B have the same bone setup.
Maybe someone more experienced can confirm this or has another solution?