Do you mean they split as you trim them or do they just appear to be split?
If it's when you clip them you might need a sharper pair of clippers. At work we can always tell when the blade gets dull from the way the cat's nails look after you clip them. They tend to look split and splintered.
If they have the apperance of being cracked(I'm not sure if your kitty goes outside or not) it's common in cats who go outside to have their nails look worn. My cat who lives with my dad, Benny goes outside and his nails are always short because he uses the trees as his scratching post.
And it may even be that he's shedding his nail and the nail itself can look like it's spliting before he sheds them.