CRUSH blueberries throughly, one layer at a time. (Or place blueberries in food processor; cover. Process until finely chopped, using pulsing action.) Repeat with strawberries. Measure exactly 2 cups prepared blueberries and 1 cup prepared strawberries into large saucepan. Stir in lemon juice and pectin crystals.
BRING to full rolling boil on high heat, stirring constantly. Boil 1 min., stirring constantly. Remove from heat. Stir in sweetener. Stir and skim foam for 5 min. to prevent fruit from floating to top.
POUR immediately into warm sterilized jars, filling up to within 1/4 inch of rims. Seal while hot with sterilized two-piece lids with new centres. Store in refrigerator for up to 1 week. For longer storage, store in freezer.