Dual boot macOS Catalina 10.15 beta and macOS Mojave 10.14
Your existing installation won't be blindly updated. The macOS 10.15 Catalina beta installer will launch as soon as it is downloaded.

You can create a bootable USB installer by plugging in a USB flash drive and running the following in Terminal:

sudo /Applications/Install\ macOS\ 10.15\ Beta.app/Contents/Resources/createinstallmedia --volume /Volumes/DRIVE


where DRIVE in the name of your external USB drive

You can choose to install macOS Catalina on an external drive, if you don't wish to interfere with your current installation.