You have to set up your two screen as separated screens in Windows by choose "Extend displays" (cf
https://www.youtube.com/watch?v=38NPxhujOxo)
Then in MadMapper projector tab, you can choose the output screen for each projector (in case you want to do multiple fullscreen outputs)
If you don't change anything, MadMapper will start fullscreen on the first output which is not the "main display" in windows.