When you have 2 external monitors of the same model connected to your computer, ManyCam can not differentiate between the two. When you select either of the 2 desktop sources, you will get the view from the desktop on the primary (first initialized) monitor.
I’m using 7.8.0.17 on the Mac
@Jacques,
Please try out newer version where this problem should be fixed. Changelog: https://manycam.com/mac_changes/