Isn't it entirely dependent on the airlines, what counts as one itinery? How could I contractually obligate them to something they didn't agree to be contractually obligated to?
(BTW: I've had multiairline itineraries many times. I think a lot of airlines are perfectly happy to do it because they can't fly domestic in that country and the domestic airline doesn't fly international. Also, there have been times when I haven't been able to, and I've been worried about it, and I got to the airport for manual checkin, and they've said "oh, i see you have an ongoing flight to such-and-such, do you want me to check you in for the whole journey?")
(BTW: I've had multiairline itineraries many times. I think a lot of airlines are perfectly happy to do it because they can't fly domestic in that country and the domestic airline doesn't fly international. Also, there have been times when I haven't been able to, and I've been worried about it, and I got to the airport for manual checkin, and they've said "oh, i see you have an ongoing flight to such-and-such, do you want me to check you in for the whole journey?")