[neurotech1]

One case is Boeing training Captains are sometimes qualified to fly with multiple airlines, and multiple types. They have to be qualified for the operating procedures of that airline. The Boeing pilots are mainly used for training roles (supervising or safety pilot observer) and don't actually fly the jet on scheduled flights.

Some maintenance contractors have pilots who are qualified and current on several types, but its likely they are Part 91 flights only and may not be line qualified for Part 121 scheduled flights.

I can't see "freelance" airline pilots being more than a rare edge case.