That was an option before admission. Now Texas is under the same Constitutional restrictions every other state is. If she wants to divide then she need the approval of both houses of Congress.
Nope, look it up. Texas was a country with its own constitution. When Texas was invited to join the union, it added a right into the admission agreement that allowed Texas, without any other approval, to split into 5 states. Means 8 more US Senators. There is nothing the rest of the states can do to stop it.
http://www.opinionjournal.com/extra/?id=110005511