actually playcanvas does support playing multiple animations.
You have to create a skeleton, with the bones you want to mask off - find your bones through node graph.
lets say this skeleton is just the upper body.
Then grab the animation clip you want to affect the top half.
grab only the nodes in the animation that affect the top half
create a new animation clip and apply this to the skeleton in update.
works!
Base skeleton plays the animation as normally(you dont have to create this skeleton), and any animations you want to add on top you can mask off this way and play.
Tried using Blend! but this was not the animation system i expected, and there is a big bug that does not allow separate animation clips to play. at the very least on my project and rig