In past years several airlines had codeshare agreements where the plane was physically segregated. Each airline had separate cabins on the plane, and controlled the seat allocation separately. It was like a partial wet lease of the plane; but those codeshare agreements seem to have all ended (they may have been required due to regulatory approvals). I even remember one codeshare that had different cabin staff and meal service. Now I am only aware of codeshares where the operating airline controls the flight fully and the marketing airline just markets the flight.