It’s very janky and doesn’t seem 100% implemented, but you can actually be 2 people controlling a vehicle. The easiest way is sharing a car/boat, passengers can right click to aim and shoot, for those who didn’t know.
Tanks also work (specifically those with secondary gunner seats like the Grant and Ha-Go). However, if I want my mate to take a seat in my tank, I have to get some of my tankers killed first - Only then is he able to take their spot.
I suggest fully enabling shared vehicles for players who are in a party together, with players taking precedent over bots. The system seems to already be in place, and it’d be fun to use, for us few who would like to do it.