[FFmpeg-trac] #9961(undetermined:new): two audiotoolbox device outputs won't play together

[FFmpeg]

#9961: two audiotoolbox device outputs won't play together
-------------------------------------+-------------------------------------
             Reporter:  sabletor     |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:               |                  Version:
  undetermined                       |  unspecified
             Keywords:               |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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 Trying to play 1.mp3 through a bluetooth speaker/headphones
 at the same tie as playing 2.mp3 through macbook's built-in speakers


 Connect a bluetooth audio device to macOS (or some other audio output than
 just the built-in one i guess)

 ffmpeg -f lavfi -i sine=r=44100 -f audiotoolbox -list_devices true -

 showed me my bluetooth speaker output was device index 1 and built-in
 output (macbook speakers) were index 3

 ffmpeg -i 1.mp3 -i 2.mp3 -map 0:a -f audiotoolbox 3 -map 1:a -f
 audiotoolbox 1

 This plays 1.mp3 on the bluetooth speaker perfectly, but macbook speakers
 are just clicking, or playing a cruddy audio, or silent. Until 1.mp3
 finishes, then the macbook speakers finish playing 2.mp3 with good
 quality.

 running

 ffmpeg -i 1.mp3 -i 2.mp3 -map 0:a -f audiotoolbox 3

 and

 ffmpeg -i 1.mp3 -i 2.mp3 -map 1:a -f audiotoolbox 1

 one after the other in separate terminal tabs plays both at the same time
 as each other perfectly.

 but for some reason the single command line

 ffmpeg -i 1.mp3 -f audiotoolbox 3 & ffmpeg -i 2.mp3 -f audiotoolbox 1

 doesn't play anything on the built-in speakers (device 3), and when they
 are swapped

 ffmpeg -i 2.mp3 -f audiotoolbox 1 & ffmpeg -i 1.mp3 -f audiotoolbox 3

 doesn't play anything on the bluetooth speaker (device 1)
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Ticket URL: <https://trac.ffmpeg.org/ticket/9961>
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