It's hard to tell.
It really depends on your target specs, and the number of vertices you have.
More vertices means more performance cost. In the case of meshes with Weights, more bound bones means more cost (this cost is multiplied by the number of vertices).
But these costs may not be significant depending on the device you're targeting. As you probably already know, PCs are pretty damned powerful.
You can get a rough, practical idea of your case by opening the Raptor sample Spine-Unity scene and duplicating that a bunch of times and seeing when it starts to slow down.
I get 60 FPS inside Unity Editor on my cruddy old Core 2 Duo from 2009 with 100 raptors. Should be even faster in an actual Release build outside the editor.
Check the Raptor's skeleton's mesh structure complexity by unfolding the MeshRenderer.
Also note that slowdown can come from other places too.
The number of pixels the meshes occupy on screen can have an effect. If there's more overdraw and fill more space on screen, it will eat up more GPU cycles.
Unity Engine has its own systems overhead that you can optimize around if you need to.
If it's only for your two characters, effects, and a bunch of stuff in the background, you should have a plenty of allowance for Free-form Deformation and Weights.