As written in the README.md:
4. How do I enable extended desktop/mirrored desktop on my X Window?
Currently Fresco Logic does not provide desktop related manipulation. Fresco Logic hopes the community will contribute to this area so that end-users can easily adopt this solution.
So, tough luck I guess. Someone needs to program the driver to be able to support X window extended desktop/mirrored desktop mode. Or if you’re lucky, someone makes some magical software that easily integrates a good number of USB to VGA/HDMI adapters.