not only shouldnt you keep another six line wrasse with the one you have, be careful adding any other wrasse. i found out the hard way when adding a carpenters fairy wrasse... i had no idea just how aggressive six line wrasses were!
also, not only are the six lines great at eating parasitic snails on clams, but mine single handedly eradicated flatworms from my tank. a great reef fish, but don't let it's small size fool you, they can be nasty little suckers towards other wrasses.