Originally Posted by
WineCountryUA
As others have mentioned the answer is no (initially). You must first check-in with the partner.
However, sometimes this does not fully check you in (such as get you on the gate upgrade list or get you a BP or you want to do SDC), if that happens you will be able to check-in with UA once the partner puts the ticket coupon for their flight. This probably will not be until after your board the partner's flight. So if there is inflight wifi or on landing you can finish the check-in with UA.
I don't think I have ever had a problem checking in with a JV partner connecting to UA, but there are more issues when the first carrier is an unfriendly Star Alliance member or a partner UA interlines with like CX on HKG-MNL-HKG. It's never caused me any problem so long as I have have my UA itinerary on my mobile device and can show to security in transit check. Maybe once these huge mergers are consumed into what we now call the US3, maybe more budget will be invested in the airlines' reservations systems so these alliances are more seamless. I find just being friendly and polite usually gets me out of most airport hiccups
