[dorrcoq]

I use Adobe Audition. You can download a fully functional trial version and use it for 7 days. Then either cancel, or if you plan to do more sign up for the monthly subscription. It's a little pricey if you aren't going to use it much.

Audacity is a little harder to work with in my opinion, but should do what you want it to do.

That said, mixing two sources together won't always result in a better sounding finished product. If one of the source recordings is inferior sounding, mixing it to the better sounding source will only lower the quality of that source.