Tex. Health & Safety Code § 286.172

An election to create a hospital district and to authorize the imposition of a sales and use tax under this subchapter, or an election to change the tax rate under Section 286.174, has no effect if as a result of the adoption of the sales and use tax or the change in the rate the combined rate of all sales and use taxes imposed by the district and other political subdivisions of this state having territory in the district would exceed two percent at any location in the district.