This heating performance chart for Dual Fuel Systems outlines the balance point for gas efficiency over heat pumps to be 30 degrees Fahrenheit. The chart below shows the monthly temperature averages for the Charlotte area. The lowest average is January of 2008 at 32 degrees.  While this is merely an average and does not reflect the actual low temperatures themselves it does illustrate that this area does not average temperatures below the balance point indicated in the first chart. 

The Dual Fuel chart above should not be relied upon too heavily in any event as it is merely comparing heat pumps to gas furnaces without qualifying the relative efficiencies of each method by the SEER or AFUE. 

Without that relation it is like saying that boats are faster than cars without qualifying their relative speeds.

Dual Fuel systems cost approximately $1000.00 more on average than a conventional system.   With dual fuel you have more components to fail, essentially you have the problems of a heat pump and the problems of a gas system plus the added control mechanisms for change over. 

I have seen no convincing evidence that these systems will save enough money over conventional gas furnaces or heat pump systems to justify the additional expenditures for these systems.