If you get Silver on US Airways, your United account will still remain the same as it was before. However, your Silver status on US Airways gets you the corresponding benefits on all the airlines of the Star Alliance (*A) network.

As jefi99 says, you only need one account from any of the members of *A to earn miles on their network. It doesn't necessarily have to be US Airways or United, it can be any one of the members' programs, whichever suits your needs better.

For instance you can get Gold status with less miles on Air Canada or Turkish Air, and if lounge access on domestic US flights is what you want, membership on one of the non-US members is what you want. For flight awards though it's cheaper to get awards on the airline whose program you are a member of.

While you already have the US Airways and United accounts t should be fun comparing them with other members programs. As a reference you may want to check out BlondeBomber's Star Alliance Gold Chart and the Star Alliance Silver Overview by FLYGVA.