I might be doing this wrong...
But I am trying to avoid having to have use two animation tracks for EVERY animation. I do however have an animation where my player equips an item and only needs to animate one arm. I found that setting this animation to track 1 "overwrites" the arm's current animations so it works perfectly. However I am having trouble stopping the animation from infinitely repeating because there doesn't seem to be a way in GM to "clear" the track so that only track 0 is playing.
The only function is skeleton_animation_set_ext() which does not accept "noone" or "null" or "0" or anything similar. For example: skeleton_animation_set_ext(noone,1)