You juste have to have 1 friend.
put a backpack in a vehicle (maybe it works on the ground too, i didn't try)
open the vehicle's inventory , your friend opens it too.
Just right click on the backpack on the same time, and you both have the same backpack on you.
when you put items in it, you friends see them too.
Now, both players just spam right click on items in the backpack, and it will put x2 items in the vehicle for each click.
It's very easy to do.

If someone has an idea to fix that, i'm interested