How can I make Windows accept more than one password as the correct password at the logon screen? What registry key do I change, or what setting?
I doubt that would be possible. It's probably possible to have two accounts share the same profile though, if you adjust the NTFS permissions appropriately. If it is possible, it would probably be best to not have the two accounts logged in at the same time.
No, this is not possible under Windows. For each user a single spot is reserved for the password hash of each user in the SAM database.
Under Windows 10 you can also assign a PIN to a user which can be used to log on locally to the machine, effectively giving you a second password that that purpose only.