I have a question regarding combining mp3 tracks .?

My question is that I don't want the mp3s to be joined end to end, I want them to combined . Like, one of them is a recording of a song I did and one of them is a recording of a song my friend did . Now I want to combine them so that an output file is created that could feature both my friend and I singing at the same time . Can anyone tell me how this could be done? I've tried Audacity and a couple of other mp3 joiners, all of which didn't do the kind of joining I wanted . Thanks im advance .