Dual-Booting macOS - Share User Folder

[KingCornWallis]

I am running macOS High Sierra and Monterey on the same drive via separate logical volumes.

I want to be able to access all the files in both OS's while logged in to one, but I obviously can't access the user folder of the other macOS unless I am logged in to it by default.

I know how to add and edit other users to a folder Sharing & Permissions, but I don't know how to add a user on another OS.

How would I go about this?

 

[velocityg4]

when multibooting. If I want to share files between OSes. I store files on a separate partition or drive.

 

[gilby101]

but I obviously can't access the user folder of the other macOS unless I am logged in to it by default.

Why not? Just need get to it in Finder. Like in this-macos-disk/Volumes/other-macos-disk/Users/otheruser.
You may have permission issues. These are eased if user-on-this-macos and user-on-other-macos were the first to be created on each macOS, because both will have user id 501.