As far as melee goes, I've gotten used to just using an "all but main window step forward" key on my dk crew. You could also take a step back with the main toon, but if they're on follow and you step back too far, the whole crew will turn around and face you instead of the mob. Doable, but you have to get used to it.

As far as 2x shaman goes, I'd still stick with elemental. I've done 3 and 5, so things still die faster than if you're alone and you don't have to deal with the melee range issue so it's slightly faster. Basically, if at level mobs are going to die with 5-7 lightning bolts, you can either throw 5 at once and follow with a shock, 2 waves of 3, or in your case 3 waves of 2.

Actually, with just dual boxing, you might want to consider other pairings, even dual class pairings. Warlock or hunter + another dps/healer like a spriest or ele shaman gives you tank +dps + heals all in one team and able to take down a ton of mobs and elites without stopping. I've leveled a lock with a priest doing nothing but being on follow and healing the lock and it's pet and it's insanely fast. Lifetap + flash heal = never ever have to slow down.