Screen sharing between two iMacs (mini display/catalina to thunderbolt/big sur)
Main iMac - 2021 version with Big Sur - with thunderbolt 3 ports.
Spare for sharing iMac - 2103 with Catalina 10.15.7 with mini display ports
I've connected miniport to thunderbolt 3 ports and tried pressing Cmd F2 with no result
If I go through finder and select screen share I get the following error
If I visit the VNC then I can access the other screen but I really just want the spare iMac to be an extension to my main Mac screen.
I run Bitdefender but don't have firewall so feel maybe its the connection mini display to thunderbolt
Has anyone successfully managed to do this?
thanks