Mushrooms, are similar to nems, in the sence that (as far as I remember) they split in two to spread... though they may/probably also reproduce sexually. When they are big/healthy you can force propegation... but I personally would recomend the waiting game, as you do risk killing the coral... and generally a healthy coral should self propagate reseasonably regularily, but if it is clearly not spreading for a long time then by all means give it a try. The Duncans spread similarily to Zoas, they create new polyps at their base, which is reffered to as Gemmination.