Depending if there is going to be various damage types you can also have other ways.
Like energy pistols could be LMB just same as the balistic version, but the RMB charges both guns for a little, then they both shoot out a much more devastating dual shot.
Explosive sidearms could for example be LMB just shoots normal bullets from one gun, but RMB shoots out small sticky devices that can cling to enemies or terrain, which don't do damage by themselves, but if shot with the LMB it explodes for lots of damage, and if you cluster them close enough shooting one will set all the others off, or you can spread them out around a THMPR so you can shoot them individually if a ground enemy gets near them.
Would make it similar to the Engineer sticky grenade launcher from Faufau, but with more skill attached to it.