The side panels are removable on the Prophet-6 and OB-6 desktop modules, so it would be possible to attach the first to the second side-to-side. Alternately, one can mount both to a thin piece of laminated plywood (longer screws) or metal sheet by way of the bottom screws (remove bottom plate on each).
If I recall correctly, the main board for the desktop modules has the connectors for the bend box and aftertouch, though these would be homed to a specific module and would not be assignable.