Besides I don't see any issue with dual client I think that banning it won't fix anything.
Anyone can put a laptop next to her/his pc screen and still keep playing two accounts at the same time.
I am an "alt-tabber" keeping my cleric in party as a free healing source but hey - I still exp these two chars way slower than single gamers who play in various parties buffing each other and decimating hordes of mobs in no time.