ffmpeg.exe -i input.mp4 -map 0:v -c:v copy -map 0:a -c:a:0 copy -map 0:a -c:a:1 aac -b:a:1 128k -ac 2 output.mp4

• -map 0:v select all video from stream 0

• -c:v copy copy this video to first video output stream

• -map 0:a select all audio from stream 0

• -c:a:0 copy copy this audio to first audio output stream

• -map 0:a select audio from stream 0 (again)

• -c:a:1 aac convert audio to aac for second audio output stream

• -b:a:1 128k set bitrate of second audio stream to 128k

• -ac 2 set channels of second audio stream to 2

As you can see, audio must be selected twice, the first time to copy it, the second time to convert and output it to a new stream.

Console will show:

Stream mapping:
  Stream #0:0 -> #0:0 (copy)
  Stream #0:1 -> #0:1 (copy)
  Stream #0:1 -> #0:2 (ac3 (native) -> aac (native))