First I have to use two separate accounts.  Then I just run WoW twice on the same computer in windowed mode and alt+tab between the two windows.  My 3.5yo computer slows down quite a bit, but it works fine.  I actually have three accounts and sometimes alt+tab between three instances of WoW so Kraygien can help the other two toons with group quests.  That slows things down to the point that it's nearly unplayable though.  I can only do that if I can set the three toons up in a way that all but one of them can remain stationary during the fight.