American Council for an Energy-Efficient Economy, Research Report U1406, Aug. 6, 2014. (Free registration required for access to full report.)
http://www.aceee.org/sites/default/files/publications/researchreports/u1406.pdf

"Compared to single-fuel programs, combined natural gas and electric energy efficiency programs often deliver additional energy and dollar savings at lower cost to utilities and consumers. They also enhance customer satisfaction. Many leading dual-fuel programs demonstrate these benefits. This report presents successful examples of combined natural gas and electric programs—residential, commercial, and industrial—in every region of the country in which there are both types of programs."