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?
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.