use the same class, same skill, control one and others just follow. search diablo 3 multiboxing on youtube should able to see some sample.

i would say if farming for rares, definately not efficient. if farm for legendaries, when you heard that drop sound, just look at which screen has the legs, then pick up, go on..

it would not be as fast as one person doing alkaizer run.. since it's slightly slower, and perhaps there's some extra work/time to do synching all characters, i would simply guess the farming efficiency is only 2 or 3 times better, not 4 times.

anyway i might be wrong, just blowing water here since server not up