Use an old laptop or tablet an additional monitor? Not a problem, and with SpaceDesk you can mix and match iOS, Android, and Windows devices (!).
And best of it all: it is (still) free.
I suspect at some stage the creators want to make some money on this app, but until then it's free. And it works reliably, as far as I can tell.
Have a look at this setup with a PC with two different monitors as well as two Android tablets and one old iPad Mini (wallpaper spread courtesy of WallX):
- works with most devices, including a very old iPad Mini
- supports more than one networked device
- requries network / wifi
- is a bit slower than TwoMon USB