Tex. Const. art. III, § 64

(a) The Legislature may by special statute provide for consolidation of governmental offices and functions of government of any one or more political subdivisions comprising or located within any county. Any such statute shall require an election to be held within the political subdivisions affected thereby with approval by a majority of the voters in each of these subdivisions, under such terms and conditions as the Legislature may require.

(b) The county government, or any political subdivision(s) comprising or located therein, may contract one with another for the performance of governmental functions required or authorized by this Constitution or the Laws of this State, under such terms and conditions as the Legislature may prescribe. No person acting under a contract made pursuant to this Subsection (b) shall be deemed to hold more than one office of honor, trust or profit or more than one civil office of emolument. The term "governmental functions," as it relates to counties, includes all duties, activities and operations of statewide importance in which the county acts for the State, as well as of local importance, whether required or authorized by this Constitution or the Laws of this State.