There are ways to encourage BTAs to split, but many are risky. The safest would be to give it a stable environment, lots of lighting, and lots of feeding.
More risky methods would be stressing it to split such as doing a large water change after many weeks of heavy feeding, sticking a tight rubber band around the anemone to basically squeeze it in half, or to take a sharp razor or scissors and just cut the anemone in half. I definitely would not recommend trying the rubber band or cutting method because of an extremely high mortality rate. Anemones are not easy to "frag" like some corals.