That won't necessarily make the meters match. The sensors can be perfectly calibrated for CO2, but still read differently because most of the sensors are sensit

That won't necessarily make the meters match. The sensors can be perfectly calibrated for CO2, but still read differently because most of the sensors are sensitive to a variety of other carbon-containing gasses. Most consumer CO2 detectors use a sensor which measures the IR absorption of its surroundings. These sensors have varying cross-sensitivities to a bunch of other gasses including carbon monoxide, propane, alcohol, etc. Calibrating them for CO2 will not compensate for different responses to CO, which is much more IR-absorbent in many of the same frequency bands.