Preheat the oven to 400 degrees F (200 degrees C). Press a pie crust into the bottom and up the sides of each of two 8-inch pie plates.

Combine rhubarb and 1/2 cup of sugar in a microwave-safe bowl. Microwave on high until rhubarb is soft and juice is pooling in the bottom of the bowl, 4 to 5 minutes. Drain juice into a measuring cup and add enough water to make 2 cups. Add cornstarch and stir to dissolve.

Pour 2 cups rhubarb liquid into a saucepan; add remaining 1 cup sugar and lemon juice. Add serviceberries and rhubarb; cook over medium-high heat until thick and bubbling, about 5 minutes. Pour into two pie crusts. Top with remaining crusts and cut holes in the top to vent steam. Pinch the edges together to seal.