Here's something I picked up from the Gnomon Videos, it may help (or not).

First of all this might be kind of buggy so you might need to delete both of the blend shape nodes, delete history and start over.

Do the blend shape for the shoulder first. When you are creating the second blend shape (for the elbow), click on the "Advanced" tab in the option box and set the deformation order to "Parallel".

By doing that you are telling Maya that the new blend shape mixes together with existing blend shapes instead of overwriting them.

Hope that helped.