Aligning groupings of objects

Was wondering if anyone here can give me some guidance on this.  

I am trying to set the bottom surface of a plastic enclosure on the sloped top of a metal chassis.   (The enclosure and chassis aren't on the same plane.)  Both the enclosure and chassis are "clam shell" designs, made from two separate pieces. 

I used the GROUP command to join the top and bottom of the enclosure, and again to join together the top and bottom of the chassis.  However, once grouped, I cannot select either surface for a coincident or parallel constraint.   Is there some other way to do this, without separating the top and bottom of each assembly?