Texas requires UM/UIM unless rejected in writing, subtracts what the other insurer pays from your UIM, and is silent on stacking in the statutes we read
Texas's rules are in Tex. Ins. Code 1952.101 (required coverage) and 1952.106 (how UIM pays). Fault is in Tex. Civ. Prac. & Rem. Code 33.001 and the deadline in 16.003.
Read how stacked and single-limit coverage differ in Texas, check what the statutes say and leave open, or see how Texas UM and UIM work.