via @macdailynews an article by @howardnoakley https://macdailynews.com/2019/07/05/how-to-make-two-bootable-macos-versions-on-one-external-ssd/
- 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 have two bootable versions of macOS on one SSD.
More details (Recommended).