If you want to try in tank, you can cut it down the middle, through the center of the mouth. This will often cause it to form 2 smaller ones, still attached to the rock. I am not sure it is worth it, because it would have split on its own eventually, with out the risk of infection.
I had a 20 gallon I used to make frags of shrooms. You can cut them in quarters through the mouth and they form complete circle in less than 2 weeks. One day an infection wiped out the entire tank. So, they are simple to propagate, but you do risk infection, and they reproduce pretty fast left alone. Normally they would not split until they had enough mass, so each half will survive. Cutting them creates tiny shrooms, and that may not be as good.