Preheat oven to 400 degrees F (200 degrees C). On a lightly floured surface, roll out one crust, and place in a 9 inch pie plate. Roll out top crust, and set aside.
In a large bowl, mix together rhubarb, pineapple chunks, sugar, and tapioca. Place in pie crust. Cover with top crust; seal and crimp edge. Brush with milk, and cut a few slits in the top to allow steam to vent.
Bake in preheated oven for 45 minutes, or until crust is golden and liquid bubbles in the center.