In the past, I found the same issue on flights to PVG: NW’s fares were much less than CO’s even though both went via NRT. I simply booked on the NW site and gave them my ONEPASS account number, and was willing to have an extra plane change (EWR-DET-NRT-PVG on NW vs. EWR-NRT-PVG had I gone CO) because the difference was about $2,500. And this was for a fare where I could use miles to upgrade.