Preheat oven to 400°. Crush the pretzels with a rolling pin. Melt the butter & mix with the pretzels. Spread in the bottom of a 9 x 13 inch pan. Bake 7 minutes, set aside to cool. Beat together the softened cream cheese and sugar. Gradually fold in the Cool Whip. Spread on the crust so it is totally covered. Place the pan in the refrigerator. Dissolve the contents of the 2 Jell-O boxes in the 2 cups of boiling water and stir in the strawberries and pineapple. Chill until slightly jelled (about an hour) . Pour the Jell-O mixture over the cream cheese filling. Return the pan to the refrigerator for several hours or until completely set.