I routinely get upgraded on NW at the prescribed 5/3/1 window (often earlier) but almost never get upgraded on CO at the prescribed window, even if the itinerary is split between them. If I get upgraded on CO at all, it is usually much closer to flight time.

CO treats NW and CO elites the same (possibly giving CO elites preference if all else equal) but most of their planes have fewer FC seats and they sell a higher percentage than NW, hence fewer available for upgrade.