If you want to remove an unwanted audio track from a dual audio avi movie file, you can use the following command with Mencoder:
mencoder original_dual_audio_file.avi -o target_file.avi -oac copy -ovc copy -aid 2
or with Ffmpeg:
ffmpeg -i original_dual_audio_file.avi -vcodec copy -map 0:0 -acodec copy -map 0:2 target_file.avi
These will create a file named target_file.avi retaining only the second audio track (aid 2).