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.
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- 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).
MacDailyNews Take: The more we learn about our Macs the more we want to learn – and this is an incredibly useful resource if you are managing multiple Macs.