I think what you were thinking of could be done. But there would need to be done by an dedicated tool.

Like if two non fullscreen windows are treated as two virtual monitors, then perhaps a lightweight Wayland compositor can be launched. Then a xwayland rootful window can be launched on each monitor, but not in fullscreen mode. As a result you have a Xorg compositor on wayland.

Such an implementation could still net some security benefits like a proper lockscreen.