42 CFR § 411.32 and Texas Insurance Code § 1301.134.

42 CFR § 411.32 Basis for Medicare secondary payments.
“(a) Basic rules.
(1) Medicare benefits are secondary to benefits payable by a primary payer even if State law or the primary payer states that its benefits are secondary to Medicare benefits or otherwise limits its payments to Medicare beneficiaries.”

Texas Insurance Code
Sec. 1301.134. COORDINATION OF PAYMENT. (a) An insurer may require a physician or health care provider to retain in the physician’s or provider’s records updated information concerning other health benefit plan coverage and to provide the information to the insurer on the applicable form described by Section 1301.131. Except as provided by this subsection, an insurer may not require a physician or provider to investigate coordination of other health benefit plan coverage.
(b) Coordination of payment under this section does not extend the period for determining whether a service is eligible for payment under Section 1301.103 or 1301.104 or for auditing a claim under Section 1301.105.
(c) A physician or health care provider who submits a claim for particular medical care or health care services to more than one health maintenance organization or insurer shall provide written notice on the claim submitted to each health maintenance organization or insurer of the identity of each other health maintenance organization or insurer with which the same claim is being filed.
(d) On receipt of notice under Subsection (c), an insurer shall coordinate and determine the appropriate payment for each health maintenance organization or insurer to make to the physician or healthcare provider.
(e) Except as provided by Subsection (f), if an insurer is a secondary payor and pays a portion of a claim that should have been paid by the insurer or health maintenance organization that is the primary payor, the overpayment may only be recovered from the health maintenance organization or insurer that is primarily responsible for that amount.
(f) If the portion of the claim overpaid by the secondary insurer was also paid by the primary health maintenance organization or insurer, the secondary insurer may recover the amount of overpayment under Section 1301.132 from the physician or health care provider who received the payment. An insurer processing an electronic claim as a secondary payor shall rely on the primary payor information submitted on the claim by the physician or provider. Primary payor information may be submitted electronically by the primary payor to the secondary payor.