This dude had a good idea, never come across these hdmi dummies before !
There are some sketchy Android apps that let you make your devices additional screens. Tried it and didn’t like something about it, I kept seeing traffic on the Windows computer even when second screen was disconnected and the app not running. One of those, if it seems wrong, ditch it.
Also there is a feature in Windows 10 that sometimes allows you to send additional screen to a second Windows device How to Use a Laptop as a Second Monitor I’ve not had great luck with this choice, but also haven’t used is extensively either. It did seem to work well going to my TV.
Think there is something called splashtop that does the same thing… Can be pretty convenient