The idea of a smoke detector which also detects carbon monoxide seems brilliant, except that smoke rises and carbon monoxide sinks, so either you place the device in the middle of the wall (and lose out on early warning) or it becomes useful for one and useless for the other.
Carbon monoxide is lighter than oxygen and carbon dioxide, about the same as nitrogen. All things being equal it will slowly rise in a room.
The reason carbon monoxide is a threat in the lower levels of your house is not because it sinks but because that's where your furnace and hot water heater are.
Combined smoke and CO detectors are fine (and there are many on the market today).
"As would have been predicted by the Second Law of Thermodynamics, CO infused anywhere within the chamber diffused until it was of equal concentration throughout. Mixing would be even faster in the home environment, with drafts due to motion or temperature. It would be reasonable to place a residential CO alarm at any height within the room."
