2x Shaman (Elemental), 2x Druid, 2x Priest (Disc or Shadow), etc...
With two, you'll have more power than single players, but won't really blow people up like 4/5 boxers.

Healers will make the most impact in a battleground.
And ranged classes are easier to play than melee.
But melee get in people's faces... which can be a good thing too.

If your primary focus is battlegrounds, strongly consider four accounts instead of two.
That's enough to blow people up... and few enough to give your team a great chance of winning the smaller BG's.



For free boxing software, my choice would be HKN (Hot Key Net)