For 2-boxing, any class combination "works". Boxing the same class just means it doesn't take much thought/preparation/effort to sync them up.

Warrior/shaman would be a lot of fun to level. Make your shaman enhancement and you'd get UR, battle shout, windfury. You'll be killing stuff so fast you won't need much healing, and even an enhancement shaman can toss out a healing wave or two when needed.