I have seen results in google flights with AC flights which click through to the United site for booking. The trick is to select United as the airline in google flights.

eg. YYC-LHR on AC, return LHR-ORD-YYC on UA

Having said that, these were all TATL flights that I searched, and I never booked one because I was unable to replicate it on the UA site with a W ticket on the return (due to the AC outbound being hidden when searching through UA)