People here have varying opinions on this. Some say you can't, some even go so far as to say you can't even book a partner-operated flight that takes you back to your destination if you credit the same mileage program.

Personally, I believe you would be able to do this, as you have two contracts -- one with NW and one with CO. Regardless of the fact that NW is operating the flights on the CO-coded ticket, the scope of the NW contract of carriage is limited to NW-issued ticket. You're not doing a back-to-back on the NW ticket, nor are you doing a back-to-back on the CO ticket.

Of course that's my opinion, coming from a very pro-consumer, California standpoint. Perhaps in Texas or somewhere you could get executed for this.

And if they saw all this activity on a mileage statement, it would probably be the consumer's burden to prove they didn't violate anything, so it may not be worth the risk in the first place without an advance ruling. And of course your ordinary CSR won't understand the nuance here, so they'll just tell you it's not allowed out of ignorance.