Wash jars and lids in hot soapy water; rinse with warm water. Fill boiling-water canner half full with water; add jars and water to cover. Bring water to a boil, reduce heat and simmer. Place lids in water to cover in a small saucepan; bring water to a simmer. Simmer until ready to use. Remove and drain jars and lids, one at a time, as needed for filling.
Place raspberries in food processor; process until smooth. Press mixture through a fine sieve over a bowl, reserving pulp; discard seeds. Measure exactly 3/4 cup pulp; set aside.
Combine 4 quarts water and 1/2 cup lemon juice in a large bowl; set aside. Fill a Dutch oven 3/4 full with water; bring to a boil. Immerse peaches for 20 seconds or until skins easily peel away; remove with slotted spoon and plunge into ice water. Slip skin off peaches using a paring knife. Cut peaches in half; remove pits. Place peach halves in the water and lemon juice solution to prevent browning; drain. Cut peaches into chunks; place in food processor. Process in batches until finely chopped. Measure exactly 4 cups of fruit.
Combine peaches, raspberry pulp, 2 tablespoons lemon juice, and pectin in a large saucepan; stir to mix well. Let stand for 10 minutes.
Bring mixture to a full rolling boil (a boil that doesn't stop bubbling when stirred) over medium-high heat. Boil for 1 minute, stirring constantly. Remove from heat; add SPLENDA, stirring until it dissolves. Skim off any foam with metal spoon.
Ladle hot jam immediately into prepared jars, filling to within 1/4-inch of tops. Wipe jar rims and threads. Cover with 2-piece lids. Screw bands tightly. Place jars on rack in canner. (Water must cover jars by 1 to 2 inches. Add boiling water, if necessary.) Cover; bring water to gentle boil.
Process 10 minutes. Remove jars and place upright on towel to cool completely. After jars cool, check seals by pressing center of lid with finger. (If lid springs back, lid is not sealed and refrigeration is necessary.) Store in a cool, dark place for up to 1 year. Once opened, store refrigerated and use within 2 weeks.
If using a pectin product with separate pectin and calcium packets, follow the package directions to mix calcium water. Use 5 teaspoons pectin powder and 4 teaspoons calcium water. Follow package directions for preparing recipe.