|Just use 20 air strikes. You will unlock it while playing the story most likely. It seems it counts for both people when one air strike is used. It's quite glitchy, as I got it when I requested a pick up for a stack of money. It also should unlock before 20 is used. If not buy more air strikes from the back menu and spam them. If one player got it and the other didn't, it should unlock after the next air strike.|
I recommend buying more fuel storage from each faction's outposts. My friend unlocked it while using artillery strikes (they are in the air strike menu after all), but use air strikes if you're paranoid.