Previous kodi used a firmware side driver for audio. The firmware has the ability to split the audio and drive two sinks.
Now, we've moved to an open source arm side driver for hmdi audio (which is better in many respects, including supporting HBR passthrough on Pi4), but the firmware code for splitting audio is not involved.
It could be possible to implement this through other means external to kodi (like pulseaudio), but I don't know how to do that.
Well, another reason for me to stay away from LibreElec 10 on Raspberry Pi.
I'm running LibreELEC 9.2.8 on an Raspberry Pi.
The Pi is connected with a HDMI-Cable to my Soundbar and from there with another HDMI-Cable to my TV.
Also it is connected to my Stereo-Audio-System with a Cable put into the 3,5mm Jack.
The Audio goes out over HDMI and the 3,5mm Jack at the same time, I don't have to switch anything.
So when I want to watch a Movie, I turn on the Pi, the Soundbar and the TV.
I've got the Sound from the Soundbar and the Picture on the TV using the HDMI connection.
When I want to listen to Music (MP3 or FLAC) I don't need a TV, so I turn on the Pi and my Stereo-Audio-System and control the Pi over my Smartphone with the "Yatse" Remote Control App.
The Audio comes from the 3,5mm Jack of the Pi an goes through a cable to the Stereo-Audio-System.
And now you're telling us that this setting is no longer possible because of a new Audio Driver ?
This is so stupid.
Why should I turn on my TV, my Soundbar or my 5.1 Speaker-System when I only want to listen to Music ?