With dualboxing pvp, you have all the disadvantages of a 4-5box team: Fear, Aoe, facing and following limitations, inability to adjust strategy quickly, etc. and you lack 4-5 boxing's main advantage - Burst. Susre you can focus target, run an assist train, but you won't have the power to instakill. And you'll be facing nothing but comps that work very well together, like War/druid, Lock/preist, etc.