An easy thing would be to do
/gml ultra_set(0, 2, 1)
which would give you the "shared ammo from pickups" coop ultra mutation. Similarly,
/gml ultra_set(0, 1, 1)
can be used to get "shared health from pickups" ultra mutation.
At one point someone was doing a mod that had all players have shared ammo pool, but apparently it wasn't finished and was generally acting kind of weird at best.