I came across the same problem and found, that lxd is not using the spice QXL spice video adaptor. I found in my Windows 10 VM the virtio VGA adapter used as graphics card stub.
So, if you are trying to increase the count of graphics cards, you shoud add another
raw.qemu : -device virtio-vga
to your VM config section, to provide you a second display in windows. Not sure if this works for linux based systems, as at least spice states here under dual display, that linux would require rather a dual headed config.
I’m not into this, but maybe for linux dual screen VM setup you have to alter the qemu config, your VM is using. The running config could be seen while VM is running in on the host
/var/log/lxd/<your vm>/qemu.conf
As the post here states, adapting the lxd/incus qemu config is a bit opinionated. But meanwhile there are ways to update or change your qemu config if really required:
look here