By federal law, Texas is the only state in the U.S. that can fly its flag at the same height as the U.S. flag. Think about that for a second. You fly the Stars and Stripes at 20 feet in Maryland, California, or Maine and your state flag, whatever it is, goes at 17 feet. You fly the Stars and Stripes in front of Pine Tree High in Longview or anyplace else at 20 feet, the Lone Star flies at the same height - 20 feet. Do you know why? Because it is the only state that was a republic before it became a state.
Well, that just it isn't it? The constitutional requirements are two; one, that Congress consent, two, that the state legislature consent. And that's just the point, Congress has already consented in the admission treaty that Texas can split up into no more than five states. All that is left is for the state legislature to decide if they want to. That makes it a unique situation. Any other state needs to convince Congress of its desires. Texas already has that.