Universal control on two iMacs not working - only handoff is

I wanted to link both iMacs 2019 (both updated to Sonoma 14.5) and use the same mouse and keyboard and it won't seem to work. I do have a second display screen for the first iMac and I was wondering if that's the issue.

Handoff works - like copy-paste between the two and web browser links also work.

Both are signed in with the same Apple ID and settings are switched ON for universal control. I'm starting to wonder if it's possible between two iMacs. If I google Universal Control it is usually between one iMac and Macbooks/iPads. I have yet to see two iMacs - but if handoff works you would think Linking the mouse and keyboard would also work.