So I'm not exactly sure why. But every time that me and my brother get into a co-op campaign, the host can see all of the weapons on their ship. But after the first battle the second player, the one joining the other's campaign will have an issue. Many weapons on the vessels if not all of them will have the error "Weapon_Doesn't_Exist" and is simply unable to do damage. But if it is controlled by the player who can see the weapons it will do damage.