How to make two bootable macOS versions on one external SSD

From the Eclectic Light Company:
Sometimes having an external drive that can boot one version of macOS isn’t enough: you want it to boot two (or even more). Once you have got past the confusing terms and tripwires in Disk Utility, this should prove fairly simple to set up.Format SSD in APFS
Partition the volume you create
Split the volume in two
Launch Terminal and type: diskutil list external
Run your macOS installers (Catalina and Mojave?)and install each on separate partitions.
Now you