No offense but they did not make it splitscreen because it would be too confusing trying to keep your cursor on your side. And besides I could care less my brother feels that if he touches the wii he will feel a sence of being demoted.
Besides the psp version is problaly a port from the wii version.
they problaly put a rail version in there because you are using the wii zapper. And when many people think of a gun that you shoot at a screen they think of duck hunt or an accade game. Look on the bright side the rail mode is "extra" content. not additional