ffmpeg batch command to take stereo wav input file and create an output file that consists of that same file twice in parallel
Solution 1:
The basic command syntax is
ffmpeg -i stereo_1.wav -af "asplit=2[a][b];[a][b]amerge=inputs=2" stereo_1_x2.wav
This will create a 4-channel stream.
%~1 and %~n1 are batch variables on Windows used as placeholders for substitution. The command processor (a.k.a. shell) will replace it with actual names during each iteration of the loop before sending the command to ffmpeg.