Consider a pair of Priests as well.

As a Disc Priest, you can Smite spam.
And passively heal whoever is hurt the most just by DPSing.

You could also dual-spec them to Shadow.
Which would let you raid with them, as a non-healer or do 5-man instances.

For PvP...
You'd likely want a mobile class so you can keep your distance.
You're not going to blow people up with two toons.
But a pair of Warlocks could apply several dots while staying mobile.
You could easily do the most damage in a battleground, but would not likely have the most kills.

Healers are where you're going to make the most of an impact in a BG.
With two toons at least.

If you want to PvP effectively, as a boxer...
Look at going with four toons most likely.

For PvE and the 5-man's.
Two toons (Tank + Healer) could work.
Or three DPS would be fine too.