A first session produces the file movie.avi with a single audio track -a 0. Now, we use a similar command to extract a second audio track from the source. Suppose, we used the example modules vmod,amod for video and audio extraction.
The second session goes as follows. This session will be much faster, since no video is de/encoded.
transcode -i (...) -x null,amod -g 0x0 -y raw -a 1 -o add-on.avi -u 50The resulting file is only a temporary file but enables proper processing with avimerge. The export module -y raw is fine, since it writes proper (0 bytes) video frames. Set video import module to null. If all goes well, both files should be identical with respect to the number of frames. Choose the proper audio track with -a. Try also to increase the internal buffers -u for speedup.
avimerge -i movie.avi -o dualaudiomovie.avi -p add-on.avi