I spent a lot of time and tried connecting two bluetooth devices to get audio from Raspberry Pi 3B+, but it looks like there is no way to do this with Bluetooth 4.2. You can connect multiple bluetooth speakers to Raspberry Pi, but only one of them will sound. The other one is just a connected ‘card’ when you do:
pacmd list-cards
but it does not appear in ‘slave’ list when you do:
pacmd list-slaves
Following will give audio output from two sources, but with a relatively significant delay: (In my case, HDMI and Bluetooth)
pacmd list-sinks
pacmd load-module module-combine-sink sink_name=MyCombine
pacmd update-sink-proplist MyCombine sink_properties=MyCombine device.description=MyCombine
pacmd set-default-sink MyCombine