I have an application which makes two uses of playing two files at the same time. The first is where it plays a white-noise file very quietly and continuously so that the sound-card does not go to sleep and therefore not 'pop' as the card switches on and off between songs. Secondly the application allows a user to start a new song and this causes the old song to fade out and hence for a short time, both songs play.
I have been using HiFiBerry DAC+ cards successfully for these two cases without issue on old Pi 2 and 3 boards but I would now like to make use of Bluetooth connected speakers and a Pi4. I have bluetooth working on buster and i can play a single instance of mplayer but a second instance is blocked because the device is in use. I presume I need something like 'dmix' and pulseaudio but I do not know where to start as some sites state that bluetooth and pulseaudio do not work well together and other sites that say its fine but I have not found anything that shows how to share the bluetooth connection (mixing the songs together).