Dear HipRock, All comments are correct. What I would further suggest is to clip the back cover of the new passport with the front cover of the old passport to carry them together. There must not be any problem while travelling to Nepal but once you land in US after your return trip it will be convenient for you to show the USVisa at the Immigration in your old passport. This way you don't misplace the passports and during your stay in US after that too they must remain together in case you need to show them at any place due to any valid reason.