The tutorial is working for me (libreelec 8.0.0) - with little changes:
stop kodi
load the udev module
...
list available sinks
(this command doesn't show all my sinks (Intel NUC DN2820)
list available outputs from alsa
add sinks for the alsa devices
pactl load-module module-alsa-sink device=hw:<CARD>,<DEVICE>
(you must replace CARD and DEVICE with the correct values from the previous command)
list available pulseaudio sinks
load combined sinks module with specified sinks (I have changed the command a little bit - the old one generates only errors)
pactl load-module module-combine-sink sink_name=combined slaves=<sink1-name>,<sink2-name>
start kodi
then select the relevant pulseaudio output from the kodi audio output selection. (the last one in my case)
I added the three pactl lines to my autostart.sh to setup the combined audio output on every startup...
It's working perfect! Thanks for the great idea!