You can have your Catalina and still be Mojave fresh.

With the public beta out for macOS 10.15 Catalina, you might be a thrill seeker and want to test out the in-progress version. But maybe you’d like to hedge your bets. In the past, you’d need to partition your startup drive, which could turn into a lot of effort, or get an external drive—preferably SSD—and install and boot from that.
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https://www.macworld.com/article/3405588/how-to-install-the-macos-catalina-beta-in-its-own-apfs-container.html