It works the same as with Demon Hunters, AFAIK: Your actual DPS becomes the average of the two weapons, since the game counts every second attack from the off hand in that scenario. HOWEVER, your ATTACK SPEED goes up, at least for Demon Hunters.
Best combo for Monks is two-handed, and two-handed weapon+quiver for Demon hunter because of this, unless you get a heck of an attack speed bonus.