It's quite obvious that most customers will want to purchase YX metal as YX and NW metal as NW, for many codeshare flights.

When pricing out the same itinerary for NS MKE-PHX on YX, October travel, the cost for the YX intineray is $337.50, including all taxes and fees; NW prices out the exact same itinerary (same YX flights, dates, number of pax, etc., at $551.50...nice mark-up!

One of the problems appears to be that the two carriers have a different fare basis for the same seats.

Unless you are accumulating flights for elite status, it really pays to cross-check the cost of codeshares, which often have a sizeable discrepancy.

I have noticed, however, that sometimes a NW flight will price out less with one of their other codeshare partners, than as their own flight.