I just cannot stress enough that smoke detectors and CO detectors are different beasts, and that fact alone argues agains incorporating them in the same device.
CO detectors are not simply "electronic." The sensing media is described by the industry as having a 'biological element.' While I have no idea just exactly what that means, the result id the same: instant alarm at freezing (so much for having one in the car or RV), and a very definite 5 year life. So, having one is an unheated garage is a bad idea.
Placement of smoke alarms has been pretty well figured out, since the 70's. I'm not quite sure what you mean by 'the radiant rise,' but no matter ... placement is pretty well explained by the instructions, and defined by code.
There is absolutely no such consensus as to the placement of CO detectors. The relevant NFPA standard simply says 'follow the instructions,' and the instructions essentially say 'put it anywhere you like.' This thread is not really the place for me to go into my ideas on the subject.
There is also an enormous difference in performance between the battery powered and line powered versions of CO detectors, and some value in the continual read-out that some models have. It's a bit hard to read that display if it's on the ceiling.
Finally .... there are also a number of things that can fool a CO detector - one being natural gas itself. These complications are another reason why I resist joining CO detectors with smoke detectors in the same discussion.
