Speak at the same time on the TV's hdmi input and bluetooth headset.

I have a bluetooth hearing aid and I want it to sound on my hearing aid as well as on my TV.
The hearing aid was successfully paired with the Raspberry piers and the sound in the hearing aid is also heard.

I also installed this:

sudo apt-get update
sudo apt-get install a2dp-app-osmc

What do I need to configure it for?

Well by default OSMC is configured to automatically switch to Bluetooth if a speaker is detected. I guess if you want to have audio on bluetooth and HDMI at the same time you would need to dig into the pulseaudio sink configuration to figure out dual sink configuration.