Its really up to you, natural splitting is obviously the best method, but cutting is just as effective. Obviously its a little more stressful on the anemone, and special care needs to be given to ensure a healthy recovery. I've cut 6 BTA's and 3 mini carpets, and all do well. There is however a technique.
I usually start with purchasing 1-1.5" razor blades, can be found at staples, ect. Remove the anemone from the tank, and I place it in a clear tupperware container. The ones that are about 1" high and 6" long. The anemone at this point is obviously closed up, when I take the tip of the razor blade and put it in its mouth, push all the way down until I can feel the rubbermaid container, and cleanly cut straight accross the anemone. I do the same with the other side.
The anemone can be placed back into the aquarium, but as mentioned earlier, extreme care must be given to ensure a healthy heal. I usually place the anemone back in the tank inside of the rubbermaid container, with the lid on, to ensure that the current doesn't blow it around, fish don't pick at its insides, sand doesn't get in ect. Keep in mind that the rubbermaid container (should of said this earlier) at this point has several slits in it to still ensure adequate water flow(you can even use a second container for this purpose)
Normal healing time begins almost immediately, where the anemone seems to almost fold over itsself, and in about 2-3 weeks, anemone's vary in recovery rates, it is usually solid enough to be reintroduced into the tank. You will notice new tenticle growth ect.
There are a few threads on Youtube, as well as Nano Reef's forums, give them a read. It is a little scary at first, but they pull through just fine. Some people's recovery techniques are different, but as long as you make sure you keep and maintain a clean and effecient cut you should do well. Good Luck