With the current set up here is a suggested way to do that:
Create a layout 'Prefab' with no event sheet.
For each different character, create a Spine object.
Place one instance of each character on 'Prefab' layout.
For each instance on the 'Prefab' layout, assign the atlas, json, skeleton name, etc.
Now, on your main game layout and event sheet, you should be able to create the above objects with the default Spine settings for each object. You could also create a 'family' for the Spine objects, so you can have some common events, behavior for them.
This all being said, Tower Defense games usually have a lot of objects, I would also do some quick performance testing to make sure that your expected number of Spine objects will run at the FPS you want for your game.