Originally Posted by House

With respect, this is almost totally untrue. Airlines use a whole variety of computer systems, and while some are shared, others are not.

For example, airlines within an alliance MAY be able to see reservations from other alliances, and airlines who use a particular booking system (e.g. Amadeus, Sabre) may be able to see other bookings from certyain other airlines using the same system.

...

What IS true is that most major airlines (apart from low cost airlines) have interlining agreements with other airlines that allow baggage to be transferred from one airline to the other. However, unless you have the flights ticketed on the same booking then there is often no way for the check-in agent to verify whether the booking on the other airline even exists. All they have to go on is an (easily Photoshopped) printout or screen on a mobile phone. Back in the days of paper tickets they could at least verify that you held an actual ticket.

Added to that, the airline whose baggage tag appears on your bag is responsible for that bag, even once it transfers to the other airline. That means they have to pay out if it goes missing, help you to trace it, etc.

.....