Cannot set "Use as" to extended display with 2 imacs
I am attempting to extend one 27 inch late 2015 imac to another. I got them to connect and the mouse moves between them, but currently both displays are set to "Main display". I want one of them to extend to the other, so I went into settings and changed the Use as to Extended display. The screen faded black for a second and turned back on, and nothing had changed.
[Edited by Moderator]
iMac 27″, macOS 12.6