I assume that you have "attached" skeleton B to skeleton A by using a
BoneFollower component and letting it follow a certain bone, as skeletons can't be attached to slots in the spine-unity runtime (
SkeletonAttachment is not a supported feature of spine-unity).
Do you mean that you have exported two skeletons A and B separately, and you would now like to play animations on both A and B in sync? If so, we would recommend to let your animations be of the exact same length on A and B and just issue the same
AnimationState.SetAnimation calls on both skeletons. Or did you mean something else?