Create a new Partition

Following How to create a partition on your Mac to install the developer beta of macOS Catalina

Please ignore How to switch partitions section

Install the latest "official" macOS on the new partition

Restart Mac, and press Command+R immediately after launch Mac. Then you can see "Reinstall macOS" option.
Please make sure that you select the new partition for installation.

Install macOS Catalina beta

1. Get profile from Beta Software - Download - Apple Developer
2. Install downloaded pkg
3. Then we can get update in Preference, continue to install
4. 🎉

refs

• https://www.imore.com/how-to-partition-your-mac