If at all possible, four-boxing is probably the way to go if your focus is PvP.
And five-boxing if your focus is PvE and/or PvP/PvE mixed.
Just because you have the firepower to blow people up in PvP, and are a full group in PvE.



Just about any two toon combo will work with two toons.
My suggestion would be to play classes that you enjoy.

Druids and Paladins are a prime candidate.
The Druid can be Melee-DPS, Caster-DPS, Caster-Heals or Tank.
The Paladin can be Melee-DPS, Tank or Heals; possibly Caster-DPS to a degree with a full group of Shockadin/Exorcism pallies... although that won't be nearly as good as DPS as any other ranged set of toons.

Druid Tanks are lacking until Thrash? (the 81st level talent).
You can run a Druid Tank, but they won't have all their tools.

I'd lean towards a Tank Option and a Healer Option.
If you were only doing two.
And personally have both able to go DPS, both ranged or both melee.
For ease questing and syncing up for PvP.