How do I measure CO and CO2 concentration in a gas?
From my brief Googling, it looks like you'll be better off purchasing an existing CO2 and CO monitor.  If you're trying to measure high concentrations, then you'll have a harder time of it.   You haven't said much about your project, so I don't know if you're trying to go back to the physics of it, design your own circuit board with that feature, or just get the measurements of indoor air.

CO meters are available for wall plug-in for as little as $25.    You could either try to look up the sensor, or just tap the LED display and let it do its own thing.  The LED display shouldn't be too hard to reverse-engineer. 

CO2 meters are harder to find and more expensive ($100-250), but they're also available.  They're used in air quality/HVAC monitoring for people and for plants, as well as in taking higher concentration measurements for toxicity analysis (ex. storage of dry ice - you don't want to suffocate by opening the closet door!) or for making carbonated beverages.    Wikipedia has an article on non-dispersive infrared measurement techniques.

There were also CO2meter.com has sensor systems with I2C interfaces (datasheet) available for as little as $99, which looks like a good price. 

A silver halide system can be used to make the measurement if the NDIR systems won't work for your application, and a Wheatstone bridge would be required to measure the tiny resistance, but, again, if you're going for accuracy, you'll probably be better off purchasing a commercial solution.  If you're trying to manufacture your own sensor system for sale, you'll need to do your own research.