There is nothing wrong with buying multiple seats on different flights. This is why CO is willing to oversell - they expect some folks to cancel.

CO has systems in place that will prevent you from booking multiple seats on the same flight (search for "super dupe snoop"), but you can absolutely have multiple reservations between the same cities. I did it recently with two different returns from SEA-EWR while I was decided between the late afternoon flight and the redeye.