The Texas Annexation is already spread across Texas and four other states: New Mexico, Oklahoma (the panhandle), Kansas, and Colorado. Texas is the only one that looks to be wholly formed from the annexation territory, but the other four include parts of such land. As such, there is no remaining right to split Texas. They've already been split as much as specified in the Joint Resolution for Annexing Texas to the United States.
From Wikipedia. The full size version of that map actually shows the Texas Annexation stretching into a sixth state, Wyoming.
But there's a way
If Congress and the Texas legislature agree, Texas could split. This is just to say that there is no remaining split from the original resolution. That split has already occurred. Any additional split would be new and in addition to the split specified previously.