Cut ribs of rhubarb in half lengthwise and slice into 1/4-inch pieces. Combine with cubed strawberries and 2 tablespoons sugar in a bowl. Toss to combine, cover, and refrigerate for 1 to 2 hours.

Combine flour, baking powder, baking soda, and salt in a mixing bowl. Whisk to combine.

Drain fruit in a strainer set over a bowl. Reserve the syrupy juices that drain through.

Grate frozen butter into the flour mixture while stirring with a fork to coat butter pieces with flour. Add 1/4 cup sugar and drained fruit, mixing until coated with flour.

Add 2 tablespoons of reserved fruit juices and pour in buttermilk. Mix until dough comes together. Adjust with more buttermilk or flour as needed.

Preheat oven to 450°F (230°C). Line a baking sheet with a silicone mat.

Scoop equal portions of dough onto the prepared baking sheet. Optionally, brush biscuits with egg wash and sprinkle with sugar.

Bake for about 25 minutes. Transfer to a wire rack and let cool completely.