To enable virtual displays on amdgpu in Bazzite edit the kernel parameters on unused ports.

To determine unused ports on your gpu use:

for p in /sys/class/drm/*/status; do con=${p%/status}; echo -n "${con#*/card?-}: "; cat $p; done

Then add the parameters:

sudo rpm-ostree kargs --editor

Add the following arguments to the kernel, changing port,resolution and refresh as required:

video=DP-2:1920x1080@60e video=DP-3:2560x1600@60e

In Sunshine you should now see the new displays. You can use "command preparations" to enable/disable displays as required.

Examples:

Only DP-3, other displays disabled (parameters can be derived from "~/.config/monitors.xml where the settings app stores settings)":

gnome-monitor-config set -LpM DP-3 -m '2560x1600@59.987' -s 2

gnome-monitor-config set -LpM DP-1 -m '1920x1080@60.000' -t normal  -LM DP-3 -m '2560x1600@59.987' -x 1920 -y 0 -s 2 -t normal