I think there is no feature of exporting/importing Test Objects implemented in Katalon Studio.
However I guess (though I have not tried yet) you can copy the *.rs files under the Object Repository directory of a project to another, as Test Objects are recorded in *.rs files. No more mysteries.
You can copy *.rs files by any method you like. Manually, bash cp command, PowerShell Copy-Item, or Gradle copy task etc. You would need to close and reopen the destination project in Katalon Studio to let them acknowledged Test Objects.
However, I would rather recommend you to manually merge these 2 Katalon Studio projects into 1, avoid exporting/importing Test Objects at all, if the 2 projects are closely related.