Hello again
another addon really useful for me.
Which it does is split a certain expression in every vertex group existing in both models (the main one and the expression one). You would not need to have the same vertex groups in both meshes. Ideally, the main one would have the whole vertex groups set and the expression one would have those vertex group that you want to get (ie, the vertex groups only related to the upper lip)
Also, it connect the shape keys to a given driver. I do not create the driver automatically in the script (I could, but I have just realized of that hehe) so you have to create a driver layout, which should been compound by
- an object (the rectangle in the video). Which you select to duplicate the whole driver
- an object child of that parent. Which will be connected to the shape key. It will be connected to the Z location transform.
You can download from here:shape keys tool
here is the explanation video: