>>279986850
By default SteamVR loads only one VR driver. You have to edit a config file to enable hybrid setups.

>>279986881
Given that ALVR manages to pipe through stuff from and to Quest, I recon the way to go about it would be to write a software that runs in Oculus mode and just grabs the inputs and pipes them to a SteamVR driver. Maybe you could even use the tracking data and just sync up the playspaces.