Quinn, thanks! I finally copied and imported .p12 certificate/key/indentity on my second Mac. Also I had to import new intermediate certificate from Apple website to recognize the imported certificate.

Quinn, please read my reply below.

I raise this question again. Earlier you suggested: "The easiest way to do this is use Xcode’s import/export feature. Launch Xcode, choose Xcode > Settings, select Accounts, select the account in question, then choose Export Apple ID and Code Signing Assets from the action (…) menu." In Xcode 16 I cannot find any import/export commands to move existing codesign certificates/keys to my second Mac. Probably it will easier to create a NEW individual codesign certificate for EVERY Mac I use?