Adding a Volume of macOS mojave, to a mainly macOS catalina computer.

FWIW, If you bought a 2019 16" MacBook Pro that came with Catalina you can't install Mojave.

With any other MacBook Pro it's simple enough to install Mojave on a second volume. You would have to create a Mojave USB Bootable Installer…

How to create a bootable installer for macOS - Apple Support

Add a second volume to your Container and install Mojave to it.

Installing macOS on a separate APFS volume - Apple Support

Are you wanting to have a partition with Mojave so you can dual boot your Mac? That would be either not possible if yours came with Catalina, or somewhat difficult having to start with an empty drive, and then installing two separate OS versions.

Since the additional Mojave volume appears on the booted Catalina Desktop, it might be prudent to do some fstab suppression work, or at least ensure that Catalina's Time Machine ignores it. Same drill on the Mojave volume.

Usually the volume is a computer setting not OS related. There is not such thing as a macOS volume setting. Applications may have audio volume preset.

WD

Thanks, I was not aware of the option in that second link.

You're welcome.

Because you wouldn't want a backup of either?

For me, I prefer individual TM backup solutions on separate drives.

I see. Well, I haven't checked but couldn't you exclude the Mojave volume from the Time Machine preferences in Catalina and vice versa. And choose different backup drives for each?

Sure. I happen to use fstab to cut down on n-partition clutter.

I see. I have no separate partitions, just 2 volumes in one container.