I just completed a trip that combined CO and NW codeshares and the CO site had it at a much lower price than the NW site. I tried several times because I wanted the NW bonuses but the price was almost double. In fact the NW site hid the CO NW codehsare unless severely pressed. So I always try it both ways. You can book and hold if need be. And the prices change constantly, so what it true one day may not be true the next.