Depending on what version driver you have installed, you may need to use a VGA dummy plug.
If you update to the latest version you wont need the dummy plug.
I actually thought about it. But dont have the resistors to make them, store closed today
I dont think newer or older driver would matter. I'm using 12.3 as well as 12.4.
The reason i think we need to use dummy (back in 11.4 driver) was the issue with energy saving feature from AMD. They disable the device if its not connected to a monitor. What they "fixed" in the newer driver (i think) was to make sure one monitor connected to any of the GPU, which didnt work in my situation as i only connect the monitor to the IntelGPU.