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Immerse 8 pint jars in simmering water until jam is ready. Wash lids and rings in warm soapy water. Combine peaches, 5 cups sugar, and lemon juice in a large stainless steel or other nonreactive pot over medium heat. Bring to a low boil and cook for 10 to 20 minutes. While peaches are cooking, combine blueberries and remaining sugar in a separate pot over medium heat. Bring to a low boil and cook until just starting to thicken, 10 to 15 minutes. Add blueberries to the pot with the peaches and cook until thickened to the consistency of jam, about 10 minutes more. Remove from heat, add butter and almond extract, and stir well. 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. 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.