u should never really need an offtank in any 5 man instance if ur needing an off tank ur doing something wrong. at most u need a healer and a support like archon/chloro most of hte lower instances got a way with just using 1 tank 3 dps and 1 heals once i hit FC i started using a chloro/archon and cleric worked out alot. all the tank classes have abilties to handle more then one target.