As a 2boxer you'll pretty much not be soloing much beyond the first few instances; lack of dps or hybrid tanks(hunter/lock pets in particular not being strong enough), then not having enough mana/health to last through the longer bossfights later on(cuz with a tank/healer setup your dps is gonna be a joke and any hybrid dps/healer is gonna be oom quicker or lose the tank to a surprise crit)