Place diced peaches and mangoes in a large bowl. Add sugar, brown sugar, cinnamon, nutmeg, flour and lemon juice. Toss till combined. Set filling aside.
Make Crumble Topping
In a medium bowl combine brown sugar, cinnamon, salt, flour and pecans if using them. Stir in melted butter and mix until all ingredients are combined and clumps begin to form.
Pour pie filling into chilled pie crust and top with crumble topping.
Place pie on a large baking sheet. Bake at 400 for 20 minutes, then with the pie in the oven, decrease the temperature to 375 and bake for another 30-35 minutes. You want to filing to start to bubble.
Allow pie to cool and set for a few hours before serving. I always chill mine in the refrigerator to allow the filling to completely thicken.
Notes
If using frozen peaches and/or mangoes allow them to completely thaw before. Once thawed, dry them with a paper towel to get any extra moisture out.I like to use a pie crust shield so that my edges to not get too crisp.This pie freezes perfect if you want to make in advance. You can freeze the pie crust and prepared filling separately for 2-3 months. Or, you can bake and freeze the whole pie. Allow to come to room temperature in the refrigerator before serving.