Originally Posted by
LondonElite
You will need to call United to get the other airline's PNR for this reservation. (Out of interest, what airline?) Otherwise the answer to your question is 'Yes'.
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.