Well, we don't know that they can--if Fray is still canon, then it's likely that the axe dealie was a one-time thing and once the current crop of Slayers die off, it goes back to being one girl in all the world.

Of course, if splitting the Slayer line once caused all that trouble, you'd think that splitting it a thousand ways would be a catastrophe. But then again, it's possible that the new Slayers are getting their power from a different source.