In other words:
There are two user profiles on a Windows 10 PC. Let's call them User_A and User_B.
A Documents folder already exists on D:.
Using an elevated command prompt...
Navigate to c:\users\User_A and delete the Documents folder. Then run the following command: mklink /J "c:\users\User_A\Documents" "D:\Documents"
Navigate to c:\users\User_B and delete the Documents folder. Then run the following command: mklink /J "c:\users\User_B\Documents" "D:\Documents"
That way, when either user is logged in and works on files in the default Documents folder the changes will be available to both users.
I realize there are alternative approaches to let them work on the same files but this keeps things easy and by default for the users.