Continental and Northwest must price their tickets independently even if the NW "flight" is actually on the very same CO metal. The government considers it price fixing for CO and NW to discuss *any* pricing of seats regardless of where the seats are.

So sometimes you end up with the odd result of NW tickets being cheaper than CO tickets on the same CO plane. Also you will occasionally find CO tickets cheaper than NW tickets on the same NW plane. The two airlines do share seat inventory on these flights though.