Can a single foreign key point to multiple primary keys?
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I have a case where the DB is set up with (simplifying) a table User and a table SuperUser. Both have a column Id. Every SuperUser row has a row in the User table with the same Id value (not my design!!!). The SUperUser table does not pull common info from the User table, it duplicates it.

Can/should any foreign key that points to User.Id also have a FK:PK relationship with SuperUser.Id? My take on this is it is at a minimum a very bad idea and that many (most?) DBs can't enforce this relationship/

Am I off base here?