Just see if you can buy them separately. Maybe it'll work that way.
Not exactly related, but several years ago, I wanted to go to Detroit from SF between two airlines, one on United and the other on Southwest (I had United credit from a LOOOOONG oversoul layover in LAS). So yeah, a flight to Chicago from SF was $350, but a flight to Cleveland from SF was $250, but stopped in Chicago.
So yeah, airplane ticket purchases are screwey. =/