Assign one foreign key to two fields in a database

[tcmauldin]

I have two tables in the database one is user and the other is comments.  I want to create a foreign key that will assign the user key to two fields in the comment table.  I get the Error message  Cannot change the involved columns of a constraint that is part of a foreign key relationship.  can some one tell me how to create on foreign key relation that will do that so I don't have to create two separate FK relations