Thanks to all of you for taking the time to reply with such extensive information and an excellent list of further reading material. What I thought was a simple question was born of more than 25 years experience with Mac HFS+ formatted HDDs and almost no experience with APFS. Having now read many of the articles you suggested I can say I have a better understanding of the many things I didn't know I didn't know and it is quite an long list.
My thought behind creating a second APFS container on the 8TB SSD was a result of the extensive backup program I have employed for the last 8 years with my trashcan MacPro. My MacPro has a boot SSD, but it is devoted strictly to holding the operating system, which at this poing is still Mojave. All of my data is stored on a 5TB HDD, except for the Lightroom catalogue files which, as I mentioned are on 4 SSDs in a RAID 5 configuration. Each night all three of these drives, Boot, Master Data, and Lightroom Catalogues, are automatically cloned to additional HDDs which are always mounted. In addition, I occasionally connect a third set of HDDs and clone my 3 main drives to them for storage offline. And since APFS was just a gleam in the eye when this system was set up, everything is still in HFS+ format.
This backup system has worked well for me for the last 8 years and has provided excellent protection for my catalogue of 120,00 images made over the last 20 years. But I can now see that going from a 256GB HFS+ boot SSD to a massive 8TB APFS SSD that will contain all of my precious data is going to require a significant rethinking of my processes, particularly my backup process. Who would have thought I would own a computer where system files and data files are closely connected by things like "bi-directional worm holes" to quote the good folks at Carbon Copy Cloner.
But those questions are for a different thread. Thanks again for educating me so well and in such a cordial manner.
Much appreciated,
Charlie