I have a Samsung Q9FN TV which allows you to either send sound to the TV speakers or a bluetooth speaker and I want to use both simultaneously. Here is my setup. My TV has a ONE Box which accepts all of the TV's inputs and there is a cable that runs from the TV into the HDMI ARC port of this ONE box. I have a cable that runs into my Xfinity cable box and from the cable box there is an HDMI cable that goes into port 1 of the ONE box. I have an RF headset.which I want to use simultaneously with the TV speakers, so I bought a D/A converter and ran my optical cable from the ONE box into the converter and then an RF audio cable from the converter to my RF headphone transmitter. I can now get audio simultaneously but only through the Xfinity cable box but not from the TV apps (eg. Netflix, Prime Video, etc). Where am I going wrong?
Nothing wrong with the setup, from what you wrote you have the audio from the ONE Box going to the headset, your issue is that you can't get audio from the TV directly which is a different thing. The TV would need the option to be able to use both at once, check with Samsung support.
If your TV has a quick settings option on the home screen it might let you turn on the speakers there and still use the audio output. I don't remember if it was a Samsung or LG that lets you do it this way but not in the main set up menu.
The only other option is to split the active audio output and connect it to a soundbar and the transmitter for the headphones.