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Ignoring the fact that it wouldn't work, it would be fun to watch.

for something that don't work it seems to work fine.
You can partition the drive but I used separate 120SSD to keep it all separate.
It's a good way to try betas. When I was googling it the main reason users create multiboot systems is because of app compatibility, the same reason I did it, and also what I thought of with Carbon Copy is what they do.
Heck you can even add a Linux and Win in there. I've used BootCamp but this way is better. I remember using rEFIt on one of the Macs also to boot Lion or Win7 and installed the hfs driver from the bootcamp package.

"I was using El Capitan without issues till now. I'm starting to use Pro Tools 10 which is incompatible with El Capitan, so I will need to install a previous OS X version like Mavericks to make it work."
"I used CCC successfully to partition up an external hard drive and install multiple Mac OSes on it, so that I could boot my computer from Snow Leopard, Lion, Mountain Lion or Mavericks."

"High Sierra was the last version of macOS that fully supported 32-bit applications. It’s likely that in Mojave, if a 32-bit app runs at all, it will have problems with performance and stability. Microsoft Office 2011 is 32-bit, as is Adobe CS 6. So, if you want to keep using those apps, you’ll need to run them in High Sierra. That means you’ll need to install macOS Mojave on a separate volume."