If people are going to run into combat, they are going to run into combat with your mage, not with the 10+K HP Warrior. If you only want to use this for PvP then there are a fairly large variety of classes you could combine, however for synergy its probably best to use two of the same class. For instance:

2x Shadow Priests - Decent DPS, Healing and Some Crowd control capabilities.

2x Warlocks - High DPS, Decent survivability, Pets, Crowd control.

2x Mages - High DPS, Excellent Crowd control, able to burn people down from range very effectively.

For the most part, people tend to avoid using melee classes in PvP Multiboxing, simply because of the extra hassle this causes. However with some thought and decent planning theres no reason why you couldn't use two Rogues to gank people (Total stunlock = win).

It all boils down to what you want to do and what classes you are comfortable with. Try out some characters on the PTR's to see what works, from that you should be able to settle on an effective dual-boxing combo.