Originally Posted by
jsloan
You check in with the airline operating the first segment. You wouldn't check in at united.com. The answer to the question is 'no.' OP can check in online using the partner's system, assuming that airline has online check-in.
Not sure about UA, but I have in the past checked in online for segments 2 and up in an itinerary with airline B, then checked in with the partner airline in person the next day for Airline A. Airline A did not offer online check in for my flight but I wanted to secure decent seats on my TATL flights free of charge at check in.
I think the answer to the OPs question is
yes and no. You can (possibly UA, definitely other airlines) check in online for later segments, but that does NOT check you in for the first segment. IME.