Can two objects linked to the same mesh data have independent shape key blends? Are relative shape key values inextricably linked to the mesh data? For example, if you had a face mesh with an "open mouth" shape key and an "open eyes" shape key, could you have two face objects linked to it with independent facial expressions? Say, Ob1 with 50% open mouth and 25% open eyes, and Ob2 with 100% both, both linked to Mesh1?
Two objects that share the same mesh data will also use the same shapekey values. Shapekeys are part of the mesh data and as such, every instance uses the same key value for each shapekey.