Immerse 8 to 10 half-pint jars in simmering water until sauce is ready. Wash lids and rings in warm soapy water. Set aside.
Combine peaches and pineapple in a large pot. Cook and stir over medium heat until fruit juices are released, 6 to 10 minutes. Add lemon juice, stirring continuously. Add sugar and stir. Add pectin; cook and stir until slightly thickened, about 2 minutes.
Pack jam into hot, sterilized jars, filling to within 1/4 inch of the top. Run a clean knife or thin spatula around the insides of the jars to remove any air bubbles. Wipe rims with a moist paper towel to remove any residue. Top with lids and screw on rings finger-tight.
Place a rack in the bottom of a large stockpot and fill halfway with water. Bring to a boil and lower jars 2 inches apart into the boiling water using a holder. Pour in more boiling water to cover jars by at least 1 inch. Bring to a rolling boil, cover, and process for 10 minutes.