Crush blueberries thoroughly, one layer at a time. Measure exactly 1-1/2 cups prepared blueberries into large bowl. Hull and crush strawberries thoroughly, one layer at a time. Add exactly 1-1/2 cups prepared strawberries to blueberry mixture in bowl. Stir in lemon juice, low calorie sweetener and pectin crystals. Let stand 30 min., stirring occasionally.