How to get multiple monitors in a VM?

I tried doing this: Setting up Multiple Monitors in a KVM QEMU VM - YouTube
But I can’t click the second monitor checkbox. So I edited the XML:

  <model type="qxl" ram="131072" vram="262144" vgamem="65536" heads="2" primary="yes"/>
  <alias name="video0"/>
  <address type="pci" domain="0x0000" bus="0x09" slot="0x01" function="0x0"/>

But it’s still not working. If I run

xrandr --addmode Virtual-2 1920x1080
xrandr --output Virtual-2 --mode "1920x1080"

I get a second screen that

  • In KDE: stays black, but where I can click and drag windows, etc.
    If I keep changing settings, the VM crashes at some point.
  • In Qtile: copies the current screen, and then freezes the image (the other screen stays usable, and the frozen screen is clickable)

Am I missing something?
What could be the cause? Could I run out of memory? If I try to give the VM more memory, it doesn’t even start. My GPU is an RX 5700XT.