Here is the page where you can see the multiple airline confirmation numbers.
Well, I kept working on this and found the answer.... I hope this may help the next one with the same issue:
It is pretty easy....
See the screen shot that I have attached....
On the United website, go to your "View Current Reservation" page for the current reservation...
Look on the page where is says:
"United Confirmation Number XXXXXX
This reservation contains partner flights.
View additional confirmation numbers.
If you click on "View additional confirmation numbers:, it will popup a window which says:
Additional confirmation numbers
These confirmation numbers have been provided for your convenience. You can use them when contacting other airlines for seat assignments or reservation assistance for this trip.
Copa Airlines Confirmation Number: xxxxxx
Avianca Confirmation Number: xxxxxx
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I was then able to contact both Copa and Avianca for seat assignments using the confirmation codes for their systems...