Hello gunspike, you can preview each skeleton with a seamless loop even at different animation lengths using the preview view:
Preview - Spine User Guide
When editing animations from multiple skeletons in animate mode, Spine will fit both skeletons in the dopesheet so it's inevitable that if the two animation lengths don't match you will see a pause for the shorter animation. If you prefer you can set a manual loop to match a desired start and end in the dopesheet:
Dopesheet - Spine User Guide: Looping