How do I connect two imac both running Catalina and connected with a Thunderbolt 3 cable?
You can't, the last iMac that supported Target Display Mode was a Mid 2014 iMac.
see > Use your iMac as a display with target display mode - Apple Support
You can't, the last iMac that supported Target Display Mode was a Mid 2014 iMac.
see > Use your iMac as a display with target display mode - Apple Support
If you are referring to Target Display Mode, that was discontinued with 2014 iMacs. In order to run TDM, if you have qualifying iMacs they both must be running High Sierra 10.13.6 or earlier. In short, you cannot use TDM in Catalina..
Any way with Catalina to use my second Mac as a dual display? (the Macs are brand new and I expected too use hem for a dual display setup.)
No sorry there is not, if you need an external display for one (or both)of your Macs, you need to buy one.
how do I connect two imac s running catalina?
