When a couple is attending a sporting event and one has the T-shirt, Jersey and/or ball cap for the home team and the other has attire for the visiting team.
Joe and Nancy went to the Yankee-Mets ballgame. Joe was a hardcore Yankee fan and Nancy was a life-long Mets Fan. Unlike other fans (who fought all night) they got along. So I guess they're a Bi-Fan-Ual couple.
A person that is a true fan of two teams that are competitors in the same sport and in the same area.
She is "bifanual" because she is a fan of the Jets and the Giants. Or the Yankees and the Mets. Etc.