Normally, you don't face this problem: If your itinerary requires a connection, you buy one through ticket. But in a few cases, you do better with two separate tickets:
-- For some trips, you have no choice: One of the connections may be on a line that does not interline baggage at all, such as Southwest, Allegiant, or EasyJet.
-- Occasionally, you can lower your costs by connecting to/from two separate airlines, especially when you want to connect with a low-fare airline.
As far as I can tell, no airline contracts of carriage deal specifically with this issue. Acceptance of two-ticket baggage on different airlines has been a policy rather than an outright commitment. Apparently, however, the three lines have decided to quit doing this, which means, except when the flights are both on one line, you'll have to claim your baggage and re-check it -- and re-check yourself -- at the connecting airport. And claiming and re-checking can create a major hassle:
-- Typically, even on a domestic flight, you have to exit security, claim your bag, then go through separate security and check-in lines at your connecting airport.
-- On an international connection, you'd have to exit security, process through incoming immigration, claim your bag, get in a check-in line, re-enter security, and maybe pass through exit immigration before catching your connection.
Two-ticket itinerary problems go beyond just baggage. On a two-ticket connection, if a late first flight makes you miss your connection, the second airline treats you as a no-show. That could mean having to buy a new ticket at a much higher fare than you originally paid.
In my experience, even airline personnel are not well informed about two-ticket policies. On a recent two-ticket trip, a check-in agent recently told a traveler I know that he couldn't check his bag through to another legacy line. The traveler finally asked to see a supervisor, who said, "Sure, we can do that." However, a couple traveling from Scotland to Quebec City on an itinerary that started on British Airways connecting to Air Transat in Paris, had a BA agent in Scotland tell them that they could check their baggage through and tagged it that way, but when they arrived in Paris, British Airways didn't transfer the baggage. So the couple had to wait to locate bags, claim them and then schlep them to a different terminal for Air Transat. By the time they got there, their flight had left, and they had to wait overnight for the next Air Transat flight, which went to Montreal rather than Quebec.
The lessons here are clear:
-- Avoid two-ticket connections unless they provide a big advantage, and
-- If you can't avoid them, allow at least four hours connecting time.
So far, I've seen no responses from other airlines. But, as I've often noted, "In the airline business, nothing catches on as quickly as a bad idea." Don't be surprised to see the other legacies jump on this one.
(Send e-mail to Ed Perkins at eperkins(at)mind.net. Perkins' new book for small business and independent professionals, "Business Travel When It's Your Money," is now available through http://www.mybusinesstravel.com or http://www.amazon.com)