Cut rhubarb and strawberries into ¾” pieces, place both in bowl, and mix well with sugar. Let sit for 45 minutes, stirring occasionally, until sugar has dissolved and there is some liquid in the bottom of the bowl.
Divide pie dough in half and roll out the two halves separately. Place one in pie plate, and pour the strawberry rhubarb mixture inside, including all of the liquid.
Preheat oven to 350.
Cut remaining dough into ½” strips. Place one across middle of pie and one perpendicular to that. Place a third piece parallel to the first, leaving a small gap, and then a fourth parallel to the second. As you work your way out, fold back every other piece to create the lattice. If you don’t have enough long strips, you can use two short ones – just have them “meet” under a strip going the other way so that the break is hidden.
Trim the edges and press down with a fork. Place pie in oven with a pan underneath, and bake for 45 minutes, until bubbly and just beginning to turn golden around the edges.