I spoke to a friend who is a CO gate agent and his *official* statement (for all its worth) is that CO will treat CO Elites higher than NW within a level, but that a higher level with either airline will earn higher priority.
Simply translated, we have :
CO Plat > NW Plat > CO Gold > NW Gold > CO Silver > NW Silver
I assume that NW reverses the priority to favor NW Elites.
Looks fair to me.