A Melee DPS and healer such as maruader + Zealot. Zealot is the easiest to box healing, as he has no mechanic, therefore no need to do special things with him or pay attention to anything except HP levels and Action Points. His Harbinger is useful, so I would hotkey that to assist your main to apply.

WAR is not very good to Software box on 1 comp. If you only have 1 comp, I would not recommend.

And.....WAR is so much fun to single box with, try playing the healing classes. They are just a world of difference compared to other games' healers. The Disciple of Khaine gameplay is worth switching MMOs for. The Black Orc and Chosen are the way tanks should have been. Their taunt is actually a critical ability in PvP and they can actually tank for their teammates in PvP (hooray for squishies!).

Another alternative, to keep things simple, would be to dualbox sorcs. Their DPS is just OP right now, and 2 could destroy anyone their level before it even reaches them. You really only need to hotkey 2 skills to do well with the sorc for DPS.