The MA allows you to have two primaries if the first one selected is an attack set. War Mace + Fire Blast is one I use in an arc. The system prevents you from using two weapon sets (redraw), having defense shields as your first pick, or having MM pets as your second pick. I think it also blocks most Blaster secondaries from being a first pick.
Thanks!! 
Yesterday I was fighting custom critters that had powers from two different primary power sets. How do you create that?