From my understanding I think this is what you want. I had a mag 60 with explosive secondary and bullet primary fire on left and right mouse click lol its a killer weapon but its been awhile though since i was focused on it xD.
This would be a script function you need to do a script, search for this through the scripts to understand.
if((self.Owner().Buttons() & BC_ALTFIRE) != 0)
Its actually applied to the ridingGun.txt in scripts folder that will help to understand some I believe look for ALTFIRE in that document.