HA! I should have known to check Piratez.. If anyone's gonna have crazy stuff, it's you
So builtInWeapons defined in units, but only the auxiliary weapon, not the primary weapon (since it is already defined in the item I guess it's obvious that it should be there).
You need to list the ammo for the auxiliary weapon as well, but not for the primary weapon.
So:
1 weapon tank: Just define in items
2 weapons tank: define auxiliary weapon + ammo for auxiliary weapon in units