Thea, When I saw the title of this thread, I imagined you were doing GS Legs followed by MM the same day!! If that were the case, I'd say NO WAY IN HECK!
But doing them the sae week could work.
Right now, I'm following my "P90X-like Hardcore rotation," which has me doing each of the Gym Style workouts 1x/week. When I move on in the rotation, I'll be using MM and HSC instead of the Gym Styles. I'd prefer to work the Gym Styles heavy, with a lot of recovery, and work MM and HSC with more moderate weights.