Sorry can't be authoritative. But just to clear up my earlier point. I believe its "back-to-back" that you're planning to do (and I mentioned earlier). This is where you get separate tickets yourself and use one when you're half way through the other.

"end-on-end" is where in journey terms you do the same thing, but the fare is constructed that way - ie you tell the airline that's what you want to do and the ticket is constructed accordingly. Of course it may not be so cheap.

I'm not sure whether airlines actually stop "back-to-back" tickets. I can imagine in the EU they might fall foul of competition rules. I once read somewhere that its mainly US airlines that try an enforce the restriction and Asian airlines don't bother.

If they were to stop it, I think they would do it via FF numbers. I'm guessing the most they could do practically would be to cancel outstanding sectors and airmiles on your account.