CO2 is Carbon Dioxide, the stuff that makes your soda fizz, this is heavier than O2, never heard of a residential CO2 detector, but if you have one, place this as close to the floor as you can.
CO on the other hand, Carbon Monoxide and is slightly lighter than O2 so you would place this as high as you can. This is why you often see combination CO/Smoke detectors which are on the ceiling.
Both CO and CO2 are created from burning something, so you will get both in a fire. But the CO is what will kill you because it will displace the good things in your blood. There are certain situations where CO becomes heavier than air, but you don't have to worry bout that since you don't live in the arctic.
