Questions about Two OS's on Mac

[exact1]

Good afternoon, I have two 512 GB internal hard drives that have over 450 GB in each. One is a SSD SATA with an APFS format and a HDD with a Mac OS extended (Journal) format. I just recently upgraded to Catalina 10.15.7 from Yosemite 10.10.5 due to other applications that needed updates (Google chrome extensions etc). I still have a lifetime license for PT 11.3.1 and a Mbox pro 3rd Gen that I would like to use on Mountain Lion 10.8. I was well aware of the the compatibility issue before upgrading to 10.15.7. I absolutely love PT 11 and I don't think I need to invest in new DAW software and equipment. So, I'm planning on installing Mountain Lion 10.8 and keeping Catalina 10.15.7. I remember 10.8 was the OS I had the least amount of PT issues with. I'm going to read and watch instructions to make sure I complete the whole process correctly. I never did a dual boot or a partition on my hard drives before, this would be my first time. I do have 5 questions. 1. Should I have 10.8 run a seperate hard drive or should I partition? 3. If running on 2 seperate hard drives is suggested, which hard drive should I use for my PT 11.3.1 the SSD or the HDD? I'm very flexible with the storage, if partitioning is suggested, I'm considering approximately 300 GB's for the Pro Tools partition and 150 GB's on the other. I believe that is a good amount to split since I use PT recreationally and barely use any storage. 4. 5. Should I change the PT partition storage to a different amount? Less or More? I appreciate the insight.

[JFreak]

If your computer is compatible with both Mountain Lion and Catalina, there is nothing stopping you to partition the system drive and installing two different OS.

But bear in mind Catalina installs a "weird partition" which you cannot erase from Mountain Lion and you should not even try, it is essential to the system. Once you want to (if ever) get rid of Catalina you need to do a little work on it.

[exact1]

I see what you're saying Catalina's 10.15.7 APFS is macOS 10.13 or later and the Mountain Lion is macOS 10.12 or earlier. That may cause an issue. I guess going to try HDD with the Pro Tools boot and see how it works.