kittens urinate and defecate at the same time, but only if their bladder is full. If their bladder is not full they will wait until it’s empty before they pee or poop. there are certain things that can prevent them from peeing/pooping on their own, most commonly food allergy.
http://wiki.answers.com/Q/When_can_a_kitten_urinate_on_its_own
Why do kittens always have to eat after midnight?