it is a very good idea. There is a Vajrayana group in my city and for many years shared the facility with a Vietnamese Mahayana group. Of course, a lot of problems can come up...scheduling conflicts and so forth (although I can't recall that too many ever did). But each group should have one person whose job it is to work with representatives from each other group. The best part was inviting members of the other group to our events and being invited to theirs. The building, a former 'fraternal lodge' hall, was rented.
Update: Today the vajrayana group and the Vietnamese group are each much larger in size and each has its own facility.
Profile Picture: "The Foaming Monk"
The Chinese characters are Fo (buddha) and Ming (bright). The image is of a student of Buddhism, who, imagining himself to be a monk, and not understanding the true meaning of the words takes the sound of the words literally. Likewise, People on web forums sometime seem to be foaming at the mouth. Original painting by P.Volker /used by permission.