Great choice of class for dual-boxing. :thumbup: I recommend dual elemental shamans since its a very fun combo and you can attack at range with both characters. I would use this spec for pve. http://www.wowhead.com/?talent#hEu0qcdItGfzAo0xxc
If you prefer to have a melee and caster combo, then make the leader an enhancement shaman and his partner elemental or resto for heals and dps support.

Good luck and have fun!