I have only made one movie using that program. Actually you cannot have two separate sound files in the timeline playing at the same time however you can have audio with the video(which is not separated from the video so in other words you can not alter its sync from what it already is with the video) and one additional audio. So for example you can have movie clips and add a song to it. You can adjust the volume of both in the tools like trashed said.
My suggestion to you would be to make the video and add one set of audio then create the movie file, then go back in and add the other. Make sure you have the loudest volume possible with the first audio becasue you will loose some when you make it into a movie and you can adjust it the second time around.
The other option would be to get another program to mix the audio in and then add it.
-------
Out of smoke then he appears
Master of disguise.