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?")
Nope, they can't, either the GDS allows booking it in which case it's already possible or the GDS doesn't allow booking it and then no app can do it. There are plenty services already which allows you to book the craziest itineraries. Like, ITA Matrix Powertools can construct a clickable link into Orbitz from ITA Matrix searches and it doesn't get crazier than that. There's Bookwithmatrix too.
