There was the X52 service from Aberystwyth to Swansea that was operated by Crosville and SWT in the mid 1980s.
The drivers swapped vehicles en route so the vehicles did the full service but the drivers didn’t. It was known for the SWT vehicle to be used in Aber on local services on occasion
When I caught the 701 Traws Cambria Cardiff - Holyhead service in the mid nineties the route was operated by National Welsh and Crosville Cymru. While the timetable showed a through journey we had to change vehicles between operators at Aberystwyth. We also had an unscheduled change of vehicle at Bangor due to a damaged mirror after getting it caught up in a hedge.