It's a common issue with lots of team play games. The other player decided that it was better to have two people operating separate mortars than to have one of them provide small arms cover on the flanks of the tank.

Helldivers 2 has a similar problem where some players can help other players reload their larger weapons at a much faster rate than typically; however the player base decided that it's better to fan out and each operate the weapon solo because shots do not need to be made so rapidly and clumping together increases the odds both players die.