Mac Studio connecting to HDMI monitor disables the Thunderbolt monitor's Display

For some reason when I connect a 2nd external monitor on my Mac Studio (max) via HDMI, my main monitor (using the Thunderbolt port) becomes disabled. It shows the screen wallpaper but doesn't allow me to interact with anything. It's just the start up screen and nothing more. I can't use a Thunderbolt port for the second monitor, because that monitor only has HDMI and the larger display port outputs.

I need the second monitor to sit above my synth keyboard, a few feet away from my actual desktop. This setup worked great for me withy iMac.

Thanks for reading this, guys,

Tom D