Dual boot macOS Catalina 10.15 beta and macOS Mojave 10.14 I wish to keep my fully working MacBook Pro running macOS Mojave 10.14, but I also want to install macOS Catalina 10.15 beta on another drive, so I can choose which OS to use when booting.

I've installed the macOS Catalina beta profile from [Apple Developer Downloads].

Now the Software Update section of System Preferences lets me update to macOS Catalina 10.15. But before completing a 6 GB download, is this going to give me the option to install 10.15 beta on a separate partition or is this going to blindly update my existing 10.14 installation?
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.