Canned food stored in a warm place near a range, a furnace, or indirect sunlight may lose some of its eating quality in a few weeks or months, depending on the temperature. Dampness may corrode cans or metal lids and cause leakage so the food will spoil. Jars do not need to be sterilized before canning if they will be filled with food and processed in a boiling water bath canner for 10 minutes or more or if they will be processed in a pressure canner. Jars that will be processed in a boiling water bath canner for less than 10 minutes need to be sterilized by boiling them in hot water for 10 minutes before they're filled. This can be dangerous because the temperature will vary according to the accuracy of oven regulators and circulation of heat.
Dry heat is very slow in penetrating into jars of food.
Also, jars explode easily in the oven. Yes, two layers can be processed at one time, in either the boiling water bath or pressure canner. Make certain that the water covers the top of the jars by 1 inch in a boiling water bath canner. The pressure canner should have 2 to 3 inches of water in the bottom. Yes, it is very important to allow steam to escape for 10 minutes before closing the valve, or placing the weight on the vent.