You should look up Emirates and Lufthansa to see what their policy is for United and US Airways. Check the minimum miles given for a segment, whether the usual coach discount fares get miles, and what are the redemption levels for awards.

About 10 years ago, Lufthansa awarded 1,000 miles minimum for every United flight, including Shuttle by United. United did not. So I credited my Shuttle by United (mostly within California and Nevada flights) to Lufthansa. 20 flights would get 20,000 miles versus 6,000 if credited to United's program.

Some airlines outside of the US charge more for domestic US coach awards.

You may decide that you want to earn all your miles in one of your programs until you reach the desired award level then stop using that program. US Airways and many domestic American programs allow magazine awards for a few hundred or thousand miles.

My guess is to don't start fresh with a United number but use your US Airways number. As far as bankruptcy, Emirates and Lufthansa are safer than US Airways. In the past, many people did not lose miles (no miles loss with Pan Am, Eastern or TWA but miles were lost with Midway Airlines and Ansett (Australia).