Something to consider with regard to the combination CO and smoke detectors is that CO is heavier than air and sinks while smoke rises. Therefore, your CO detector needs to be low on the wall, as close to the floor as possible, while you want your smoke detector close to the ceiling, or ideally on the ceiling.

A combination detector is an unwieldy compromise, because if you place it high up, your entire house can fill with CO and kill you before it rises to the elevation of the detector. If you place it low, a fire can start and fill most of the house with smoke before it reaches down to the level of the detector. IMHO, you are better off with two separate detectors, each placed for earliest warning.