Step 1Make the strawberry gelatin according to the package directions up to the point of chilling.
Step 2Meanwhile, soften the plain gelatin in 1/4 cup of cold water in a small saucepan. Bring to a boil over medium heat. Remove 1 teaspoon and stir into the sour cream; set aside. Pour the remaining gelatin into the prepared strawberry gelatin. Stir in the strawberries and pineapple.
Step 3Pour half the strawberry gelatin into the mold and chill until firm enough to spread the sour cream, but still moist to the touch, about 1 1/2 hours.
Step 4Gently spread the sour cream mixture in a thin layer over the almost-set strawberry gelatin. Chill 10 minutes. Gently spoon the remaining strawberry gelatin over the sour cream layer. Chill until completely set, at least 3 hours. Unmold and serve.