I just booked a multi-city ticket for myself and the mrs. on ual.com. One leg of the itinerary is operated by US Air. There was a line on the form for my frequent flyer number for United, and then another line (which I think was opposite the US Air flight) which defaulted to "United" and had a blank for the number. So I put in my United number again, thinking that they were asking if I wanted to get United miles or US Air miles for the flight. But I got an error message "Frequent flyer numbers cannot be the same"
So I removed the 2nd number, but left the "United", to buy the ticket.
Is anyone familiar with this form? I'll be sure to show our United numbers
for the US Air flight, but would appreciate anyone sharing a problem with
this or telling us not to worry.

I'm not sure how US's BP's look (haven't flown them in years), but as long as your MP# or something like LH's "UAMP *G" (again, not sure of the exact syntax) shows up, you *should* be fine.

If for some reason there's nothing denoting your UA # on the BP, check with a TA or GA at the airport, and they can add it.

Lastly, hang on to your BP's (or print to pdf) until the miles post.

Hope that helps!