It is possible to have Mojave and Catalina on the same Mac if they are installed on separate disk volumes. APFS can do this with 'soft' partitions that don't require reformatting the physical disk. Use 'System Preferences/Startup Disk' to decide which volume is used as the startup volume. I have been doing this since macOS High Sierra to support my clients' apps that still work on iOS 9.
SteveCaine
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