In my prototype game I'm utilizing a 3D environment with 2D character sprites. Basically, everything aside from the characters are 3D. Is it possible to attach a 3D object in Unity to an attachment slot on a Spine skeleton? For instance, my 2D Spine-animated character could potentially pick up a 3D chair model from the environment and I could set the attachment for the hand slot to the chair and the chair would behave similar to how a 2D sprite attachment would behave?

Oh wow. So, when creating an animation for just one of the skeletons Spine is smart enough to know how to keep them separate? Or, I suppose as long as I only highlight the skeleton and bones I'm actually animating with then it shouldn't matter.

I'm working on a project that will require 2 human models to interact with each other - often times 1 human will be performing an animation and the other human will be performing a separate animation. Syncing the 2 animations so they're timed correctly and are in the correct spots is tedious. Is there any tool for playing 2 animations at the same time so you could see the approximate timing of both?

Or, is there a good way to use an overlay or background image to approximate this better?

I understand that I can't save a project or anything like that in the trial version - but it appears exporting is allowed. When I click the export button I get the export dialog but when I click the final export button at the bottom of the pop up window nothing happens in the directory I have designated as the output folder.

I see no errors or anything that would indicate a problem. Upon clicking the export button in the pop-up window the window goes away and nothing happens.

This could be accomplished using skins. Check out the goblins example that comes with the spine-unity runtime. It does exactly this.

Say for instance I have 2 human sprites. I'd like to use Spine to animate a throw move - where sprite 1 grabs sprite 2 and tosses him over his head. To make this look as good as possible I'd need a way for sprite 1's right arm and right leg to overlap the entirity of sprite 2 while leaving the rest of sprite 1's body parts behind sprite 2.

So, the draw order would be:
1) sprite 1's right arm and leg
2) sprite 2's entire body
3) sprite 1's body (minus right arm and leg)

I haven't found a way to do this using my Spine-animated sprites with Unity. I've seen where I can specify an overall draw order based on the layer for the entire sprite, but nothing that would allow me to specify individual draw orders for each section of the sprite.