I've been tricked into thinking my BTA was splitting, but it's never actually split, when I thought it was going to (and mind you, I've had 5 babies with this thing...) It happens every spring, one year, I got a 3-for-1 split. Everytime it happened, it's happened overnight, and there was no noticeable warning the night before. It usually takes them a few days to heal over, and completely separate. What I've noticed is, the creature will separate, and then stay very close together, touching, for several days until they can heal over, and then one usually moves away from where the original one was.
Best of luck with the anemone, I hope it splits for you. I've noticed over the years, that generally they tend to go in the Spring, and also some split in the fall. The Message boards and Mail-list's all get hit with "My anemone just split" messages around the same time every year. It seems more prevelant in the Spring, but I've noticed a trend to have a few go in the fall as well.
- Mac