Thanks for the kind words everyone!
Edit - FYI one useful thing to know is that you can clone your Mac hard drive (e.g. using a program like Carbon Copy Cloner) onto an external USB hard drive, and then install a new OS onto that HD. You can then use System Preferences --> Startup disk to bootup in either OS.
Anyone here actually try to install a dual boot (i.e. macOS Catalina/Big Sur) and is currently using it w/ BIAB? Or is this one of those, "easier said than done" things.
EDIT: I don't have huge amounts of free space on my hard-drive either and BIAB is already running via USB external drive (that's how I purchased it).
I seriously don't want to hoop my MacBook Pro Big Sur (since its working pretty good w/ exception to BIAB). I've googled and have found general information (mostly from companies that want to sell you their cleaner product or copy/cloner product). Any links or videos someone use?
Not sure I still have macOS Catalina download link anymore.
It's not hard at all to install a dual boot with an external hard drive, as Andrew suggested. The short version is basically download it from your app store account, run the installer, and it'll ask you which drive you want to install Catalina into. Then, whenever you want to switch from one OS to the other, you hold down Option when you boot the computer. I've got an external SSD with Mac OS Catalina, Mojave, High Sierra, and Windows 10 on it, and only the Windows was difficult to set up!
If you don't have a lot of internal space, you'd need an external drive of some sort.