Enforce referential integrity for FriendRequest table that has UserA and UserB

I have a table called User that has UserName as a primary key.

I want to establish a relationship with enforced referential integrity between User and FriendRequest.

FriendRequest has the following: RequestID, UserNameA, UserNameB, Accepted. UserNameA is the sender of the request and UserNameB is the recpient. Accepting requests sets the relationship between A and B to friends. Rejecting deletes the row. No answer means that Accepted is set to FALSE.

The problem is that I can't enforce integrity and without it I can enter any user name for UserNameA and UserNameB even if that particular UserName does not even exist on UserProfile.

Is there a way to make my FriendRequest table work in Access?