Tex. Gov't Code § 842.106

A person who is a member of this retirement system and another state or local retirement system authorized under Section 67, Article XVI, Texas Constitution, may receive a benefit from this system only to the extent that the amount of the benefit is computed solely on the member's accumulated contributions and service credit in this system. Service credited by another retirement system may not be used to determine eligibility for a benefit in this retirement system except as provided by Chapter 803.