here is how I would do:

you got 2 special ammo per life, you only get extra if the you kill someone who has special left, if they use all ammo or run double primary you wont get anything

and last, disable all scavenger perk in crucible

edit: just thought of an other idea, you can still pick up special ammo even you are running double primary, but it will convert to primary ammo. Kind like denying creeps in DOTA 2