Since this seems an active topic today, I'd like to ask my own question.

Can one drop a middle leg if its on a different carrier?

For example, outbound is A->B on Carrier 1, B->C on Carrier 2, Return is C->D on Carrier 3, and D->A on Carrier 1.

If I wanted to drop C->D and drive C->D instead, can I do that without having D->A dropped?

I've done this before in Europe on a single carrier. I flew LH out to Frankfurt, took a connection to Vienna, but instead of connecting back for the return, flew from Prague instead.

This was a few years ago so I don't know if the rules are the same, and how they work with multiple airlines.

Should I just call the carriers and ask?