I found this question Questions bout Spine Features and Use. and it is weird that it is unanswered but it got my attention on this bit
Speed of the timeline. I tried to animate the body with 120 frames and the hair of the character with 240 frames, so i was trying to make a new animation for the hair but i couldn't play both animation in the same time.
The current behaviours is that if a row in the dopesheet has the last keyframe that is not in sync with the end frame of the whole animation, the period between two frame there is nothing animating (for that row) if that makes sense. Most of the time I try to make the durations divisible to other so then I can copy and paste the keyframes, that wasn't always possible and you have to change duplicated keyframes many times. The other method is to move those keyframes to another animation and use the track system, but that is efficient when using the editor because you cant see the whole thing.
I wonder if some of the AE expressions are implementable like loop in loop out and pingpong by having each "row" their own timelines that can be interpolated with their own start frame and end frame
