Set your oven to 350°F (177°C).
In a large bowl, combine the peaches and cherries with 1/3 cup (67g) of granulated sugar. Allow the mixture to sit and macerate.
Place the 9 tablespoons of butter in a 9x13-inch baking dish. Put the dish in the preheated oven and let it melt for about 10 minutes.
While the butter is melting, in a medium mixing bowl, whisk together all-purpose flour, baking powder, salt, and the rest of the sugar (2/3 cup (134g)).
Add milk and vanilla extract to the dry ingredients, stirring until just combined.
Carefully remove the hot baking dish with melted butter from the oven. Pour the batter evenly over the melted butter.
Using a slotted spoon, layer the macerated peaches and cherries over the batter. Discard any remaining liquid in the bowl.
Place the dish back in the oven and bake for 55 to 60 minutes, until the batter is golden brown and the fruit juices are bubbling.