I would like to do a clean install of OS Mojave but keep OS High Sierra on an external drive.

I have a 2012 MB Pro with an 500GB internal SSD, 16GB of Ram and a Time Machine as well a Carbon Copy Cloner back ups on two seperate ext. drives. Here is my question. I have a spare blank ext. drive that I would like to mirror my current internal drive if that is possible. Then I would like to do a clean install of Mojave on my internal drive but be able to plug in my external with High Sierra in case there are hiccups or I need to retrieve a program or information that was lost due to the clean install. Is this possible and if so, what is the best way to do it.