Time Machine restore between different MacBook Pros?

Connecting the other machine's Time Machine disk and trying to enter Time Machine to selectively restore files in that interface might be tricky. Although you say the machines are "identical," the user ID numbers might be different and that might complicate it.

One thing you could do is completely wipe/erase that imperfect Mojave portion on that machine and then use Migration Assistant to basically make it identical to the last Time Machine backup of the other machine's good Mojave portion. That's basically what one does when one acquires a new computer, migrating over another computer's applications, users, files, etc.

I'd make a second backup and test it before embarking on something like this.

Now I understand, you are using one computer as a complete backup to the other. Must take a bit of attention to keep them "synchronized," although there are cloud arrangements and software tools to do that.

I would encourage you to keep your separate versioned backups (like Time Machine) going since a problem or error could propagate to the other machine without you realizing it if you keep them synchronized.

briantho wrote:

Having started programming commercially in 1967 I've seen some catastrophic things happen thus my obsession with backups, redundancy etc. Internet connection here in Switzerland is super reliable and I do have stuff in the cloud but there's practically always a copy off site. Moi, paranoid?!

Not paranoid, in my view. I have worked in software my entire career and have also seen disk disasters, or human errors, where backups were the savior. One impressionable one was a disk pack failed, the backup was inserted into the container (this was back in the 1980s with large disk drives) and it failed immediately too. Because contamination was the cause of the first failure and the dust/particles caused the second platter to fail also. Hence destroying the backup disk. So yes, multiple backups, including different locations. Having duplicate Macs would keep your workflow going in the event of a catastrophic failure of one.

briantho wrote:

I have a question re. the Time Machine: if I run Time Machine under Catalina ONLY, does that cover my Mojave container too so that if anything untoward happened to the Mojave container then I could restore from that backup?

Or, to make sure, should I run an independent Time Machine periodically within Mojave? (I have lots of spare space)

Many thanks in advance for any advice.

You can configure what Time Machine backs up (includes or excludes) in Time Machine Preferences.

My understanding is that APFS Containers are similar to what we used to call partitions. So you would need to make sure Time Machine has been configured to back up everything if that is what you want.

I don't know if there is a complication from the fact that your Time Machine backup disk is probably HFS+ while you have multiple APFS containers on your source drive.

I think best practice would be to maintain two separate Time Machine backup physical drives, one for your Catalina drive backup, and one for your Mojave drive backup. This may also minimize complications when/if you upgrade to Big Sur.