This may be totally incorrect but IIRC if you use an app or website like CheckMyTrip, by inputting your main booking ref you also see the other airlines booking refs in the detailed breakdown
Alternatively in the itinerary/ receipt I have previously located all reference numbers for all the airlines I am flying on "one" ticket. I may not be able to check in all on one site but then have the individual refs to go to appropriate site
I am fairly certain I have successfully found mixed references before this way but the exact details I can't quite remember