Since I have been flying on mixed itineraries where UA is outbound, CO is return I have put my OnePass # on the CO segments and put the MileagePlus # on the UA segments. Which in theory works, but YMMV. For some reason the UA PNR has picked up both although when I went to check-in it displayed MileagePlus on both boarding passes.

In the instance that I cleared both upgrades in advance, I went to the Easy-Check in and changed my MileagePlus # to OnePass. On segment 1, it will spit out a boarding pass with the OnePass but on segment 2 the boarding pass will show no status, nothing. I went to the agent and they manually entered it and printed a new boarding pass with OnePass. Imagine my surprise when I look at my statement a couple of days later and the mileage was credited to UA.

I don't like having to use a UA number for UA flights and a CO number for CO flights....we are all going to be dumped into one program so while it may seem beneficial now for clearing on each carriers metal by using the respective carriers FF Elite status, its just a pain in the butt.