The only combination gains I have heard of was the one realized at the Tuskeegee Institute, where the mantle equipped kerosene lamps that the students used to study, were used to begin cooking the next day's meals. The lamps were arranged in a circle in a common room and a specially made oven, insulated with straw, was lowered down over the tops of their chimneys, to heat the inside which was filled with food for the next day's meals.

Other than this, a general rule goes something like this, compare apples with apples, when you are considering purchases or savings. Various heat sources are more efficient at heating than the odd incandescent light bulbs scattered around a room. If you want to decrease the use of electricity for lighting, buy and use fluorescent lighting; if you want to decrease the amount of energy used in your home, buy and use energy efficient heat sources, or devices like fans that direct the heat to where it is needed. Or simply buy sweaters for the entire family, so they don't mind the cooler temperatures. This worked well for us when we were in England, where heat was expensive.

Dave