Weight painting separate Meshes as one

(This is a double post,please delete this post, do not reply if possible. The Original one is here: https://blenderartists.org/forum/showthread.php?436496-Equal-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!

One way is to set the weights the same value on both vertices via the Vertex Groups section