Can you explain it better? Is the split vertical? Or, they tear across horizontally?
Anyone who has trouble with brittle or splitting nails my advice is keep the side edge rounded, even if you want the top square-ish. Also, keep them painted. Never let the time between fresh paint go longer than a week. You can use simple clear. Do a minimum two coats.
Many people say taking biotin has helped their nails and hair tremendously. I also have read iron helps, but obviously be careful with iron, too much can be dangerous, so you would want to get it checked by a doctor before you take a lot. Simple test next time you happen to be at the doctor. If your doctor usually runs blood test, most likely you would know if your iron was low.