two audiotoolbox device outputs won't play together

[sabletor]

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)