Shell and Texaco never merged. They had a joint agreement called Equilon where they shared refineries and distribution of both Texaco and Shell brands in the U.S.
Chevron bought all of Texaco October 10, 2001. ChevronTexaco got the exclusive rights to the Havoline brand name on August 14, 2003 as the licenses with Equilon expired. ChevronTexaco got the right to sell gasoline under the Texaco brand name in the U.S. on July 1, 2004. ChevronTexaco and Shell have agreed on which company gets the former Texaco stations, how fast they are being rebranded, and other details.
If you see a Texaco station, it is probably ChevronTexaco unless it is still waiting for rebranding to Shell--these things take time.
Ken