Was the ticket issued on UA stock? If so then my experience has been that mobile/counter check-in with UA should pull up all boarding passes. This was the case when I travelled from YOW to HKG (via EWR, NRT) and YYZ to SIN (via ORD, NRT) with me having UA boarding passes for the those NH operated intra-Asia segments. So long as the trip begins with a UA segment you should have UA spit out all of the boarding passes.

As another example, yesterday I had a flight YYZ to FRA (via PHL). Flights were ticketed on UA stock but first segment was with AC and second with LH. I was able to check in online with the first carrier (AC) who issued me boarding passes for both flights (although it somehow managed to screw up the FF information on the LH flight requiring me to get it sorted at the gate). That is something to be careful of when you have itineraries which cross operating carriers!

The only exception to this rule I found was when you were doing something bizarre like changing airports such as when I went HKG to YOW (arriving in HND departing from HKG same-day). At that point you need to do a counter check in because the agent has to be 100% sure you know what you're doing before they can check you in and even then they have to call a supervisor to authorize the check-in.

Safe Travels,

James