I don't believe that's the case. They're separately traded public companies. The other issue is that Valero spun off its retail business (aka CST Brands) over a decade ago. They did transfer locations because Couche-Tard (which owns Circle K) bought them out and there were antitrust concerns because of how big they were.
Retail fuel station and convenience store operator Alimentation Couche-Tard Inc. has agreed to divest up to 71 retail fuel stations with convenience stores to Empire Petroleum Partners in order to settle Federal Trade Commission charges that ACT’s proposed $4.4 billion acquisition of competitor CST Brands, Inc. would violate federal antitrust law.
The divestiture order requires ACT to divest 70 CST fuel stations to Empire, and to give Empire the option of acquiring an additional location owned by ACT. The
fuel stations to be divested are in Arizona, Colorado, Florida, Georgia, Louisiana, New Mexico, Ohio, and Texas.