The way I heard it explained: FOs were never allowed to do engine starts because senior management did not want the added training expense of getting that procedure approved by the FAA. As FOs were prohibited from doing engined starts, the XJ procedure for SE taxi was to pull aside from the taxi line, if needed, set the brakes, and run the applicable items from the before and after start checklists parked with the parking brake on. No one wanted the hassle, especially as the company refused the hassle of training FOs to start the second on the roll.