There are reasons that the alarm trade is usually separate from the electrical trade; issues like these are part of the reason.
As far as the NEC is concerned, there is no requirement for smokes to be on a dedicated circuit; other things can be on the circuit, and that would include CO detectors. Indeed there are many units out there that alarm to both conditions.
The building codes, however, stipulate that one smoke alarm will make all alarms sound; this is easiest done by having them on a separate circuits.
The "UL restrictions" referred to earlier would ONLY apply to a central-station alarm system- and not to your usual houshold smoke (or CO) alarms.
It is pretty well established where you should use, and mount, smoke alarms. Such is not the case with CO alarms. There is no reason to assume that the smoke alarm location is a good place for the CO detector.
