They are one of the more difficult corals to frag. If you are feeling pretty gutsy then give this a try.
Put a rubber band around it so it is pinching the place where you want to do the cut. The rubber band should cut through the skin over a couple of weeks or less depending on how tight it is. As it cuts through the skin it will heal itself at the same time. Once it is all the way through you can cut the skeleton with a fine tooth band saw, tile saw or dremel with a large diamond blade. viola two open brains. You may want to cut it into several pieces each with their own mouth. If that is the case wrap multiple rubber bands around it.