Click the "Video" field and select an AVI DivX/XviD movie with one audio track.
Select additional audio track
Click the second "Audio 1" fieldand select an audio file in AC3, WAV, MP2 or MP3 format, to be added as a second audio track in the AVI file. If the audio file is not MP3, it will be automatically converted to MP3 @ 128 kbps (allow 30-40 minutes for the conversion).
Make sure that the audio file has exactly the same duration as the AVI movie (eg. if you try to add an audio track coming from a PAL source to a NTSC movie, it will probably have a different duration, so leading to obvious audio sync problems).
Create AVI with two audio tracks
Select "AVI +audio" in the Mux as.. pulldown menu.
Click "Mux as...".
The resulting file will be named with a ".muxed.avi" extension.
Please note that the size of the resulting AVI file will increase of about 60MB for each hour of added audio.
To playback an AVI file with multiple audio tracks on your Mac, use VLC.
If you use a DIvX-compatible DVD player, check your player documentation to verify if it is able to play files with multiple audio tracks.
When you created the AVI with 2 audio tracks, you can add even more audio tracks by repeating this process again.