Thanks for your question. The answer depends on what you mean by “double,” and even on how we define “detector.”
Usually, “detector” means the sensing element within the instrument. But, people sometimes refer to the entire instrument as the detector.
Now, if the sensor for carbon monoxide also responded to propane, this would not be desirable, as the propane would represent an interference, and cause errors in the CO measurement.
However, if you want to find an instrument that can measure both CO and propane, correctly, and without interferences, there are several suppliers (although not Interscan) who provide multi-gas units, that would employ an electrochemical sensor for CO, and a pellistor type sensor for hydrocarbons–and that would include propane.
If you will be using this type of instrument, make sure that the hydrocarbon sensor is calibrated for propane.