Create clean install of MacOS, yet still have access to old install My goal is to create a clean install of MacOS, yet still have access to the old install (somewhere else like another partition) while I work on the new install to match the old one. I am assuming that I will miss some items initially and will need to refer back to the older install. 

One path to accomplish my goal would be to have a dual boot (though not preferred). While I am working on the clean install, I would be able to reboot to the older version to perform work if necessary UNTIL the new clean install is ready. Eventually I would delete the older version.

Another path could be to create a virtual box install of High Sierra, time machine back up that, while I work on making that right I would still have access to the old install (host). Then once I'm convinced that I have everything ready I would backup and restore the new virtual install as a real install on my Macbook.

Does anyone have any advice to accomplish this?
My goal is to create a clean install of MacOS, yet still have access
  to the old install while I work on the new install. I am assuming that
  I will miss some items initially and will need to refer back to the
  older install.


This is the very essence of a (full) Time Machine backup and the proper application of Migration Assistant.

Another option is to use an external USB drive and make an image of your existing drive to that external drive.  Once you are certain that you can boot from it and operate properly, you can then do a clean installation on your internal drive.

To make a clone of your drive, you can use a commercial application like Carbon Copy Cloner or use dd in Terminal to make the copy:

dd if=/dev/diskX of=/dev/diskY bs=1M


Where X is your source (internal) disk identifier and Y is your target (USB) disk identifier. 

Whether you use a commercial piece of software or the terminal command you need to make sure your target disk is the same size or larger than the source.