Sync Deformations on separate Meshes

Hello there everyone!

this is my very first post so I hope I'm not breaking any rules. I searched for similar topics before but I think I don't know the terminology to directly address this issue. I am very new to rigging/animation and although I think it's a very basic problem but somehow I cannot find any proper approach to it.

So the problem is this: I need to have a quadruped and a separated face/eye part with the same amount of deformation.

The model and the eye/face part are to be UV mapped separately to allow different facial expressions through textures instead of changing the texture on the entire model for each facial expression for a game. There is no problem if this were one big model and then rigged. But that wouldn't allow a separate UV Map for the face. So if I rig the model with the face detached however the edges aren't connected and show the different deformations when I start posing the head.

so basically how do I make both face and model go through the same deformations even if they are separated? I faced a similar issue before and couldn't figure out any way to solve it. Or this even an issue related to rigging? Could this problem solved be at a different point or section?

I really hope someone can help me because the entire project depends on this working out!