A great opening number for a cabaret act that is upbeat and lively is "Nothing Can Stop Me Now!" from THE ROAR OF THE GREASEPAINT - THE SMELL OF THE CROWD. Some friends of mine used it as an opening number and it set the perfect mood for their show. It's also a good title for an opening number. Not sure if a soprano voice will work but if she has a chest voice it could.
Be advised that two people can sing any song ever written together. The song need not be composed as a duet. Use your own creativity. People are giving you great choices and you are shooting them down.
Nothing Can Stop Me Now does not need Ooh's and Ah's you know that -any two people can sing the lyric and the message is good.
Tonight and I Wanna Be Seen With You are both uptempo with a great feel and message for an opening.
Magic To Do from Pippin can be sung by two people- ANY song can be sung by two people. Use your own creativity. It is not rocket science.