Prepare cake mix as directed on box for 9x13 pan. Let cool completely. I like to make ahead and refrigerate, covered.
Wash and hull strawberries, let dry til damp. Cut in half, or quarters if large, and place in large bowl. Wash peaches, smoothing fuzz with hands under running water. Let dry and cut into 1/2 - 1 inch chunks. Put in bowl with strawberries. Add 1/4 cup sugar and stir gently to distribute. Store in fridge a few hours to allow to make it's own sauce with the added sugar.
In another large bowl, make vanilla pudding with only 1 cup milk, mix as directed on package. Carefully fold in thawed whipped topping thoroughly. Can see if completely mixed because pudding will be different color than topping. Store, covered, in fridge until time to serve.
Cut cake into squares. Can be sliced in half horizontally if refrigerated. Scoop strawberries and peaches with some juice over cake. Top with combined pudding/whipped topping mixture. Repeat if sliced in half for a double decker version.