I have two separate meshes: teeth and face. I would like to apply one shape key for the face and teeth as well. When I select those two objects and hit “Apply as shape key” it only adds shape keys to the last object. In my case, it adds shape key only to the face. So, when I control the slider for mouth opening, the teeth are left aside.
Yes, you have to apply as shape key to both objects. Select face, apply as shapekey, select teeth, apply as shapekey.
After that, you have to set the shapekeys individually, or else you need to use drivers to set their values (either driving one shapekey from the value of the other, or driving both from some third control.)
If you can, it’d be best to think about if there is some way that you can join your teeth and your face. Especially if you’d like to make a lot of these.