Since your CO and DL programs are redundant (since CO and DL are mileage partners), you want to put future accumulation into one of the programs. For the other one, do your best to use up the miles on award flights or upgrades so that you get some value out of them.

If you want to use up the CO miles, but prefer to fly non-CO, try to use the CO miles on partner flights (DL, NW, etc.). Similar if you want to use DL miles on non-DL flights.