Recently I was at a large water fight, where everyone was meeting up for the end of the school year, so about 40 people went. I recently posted a new water gun I had just finished which was a piston gun. During the water fight, the piston extended too far for me to push, so I asked a friend to help carry it and aim. Even though the reason it was used by 2 people was because the gun I built was leaky and had problems, using 2 "troops" increased its range by about %70 and aiming was a lot easier, since my friend only had to aim, not shoot.
I believe 2 man "machine guns" could be used in some sort of tripod mounted water gun where there is a heavy duty hand cranked pump (Yes, I know hand crank pumps aren't practical but with another person cranking it could be worth a try) and a gunner. This would mean that the gun could have high output and high ranges without sacrificing accuracy because the gunner or aimer would not have to crank and would simply just focus on getting the target and loading. Even if this sacrifices mobility, using 2 people to power a gun could compensate for lack of movement with heavily increased range in piston guns, for example where one person could use all his or her strength to push a plunger.