In a large bowl combine apples, rhubarb, sugar, flour, and cinnamon. Cover the bowl with plastic wrap and set aside while you make the crust.
In the bowl of a food processor, combine flour, salt, and sugar. Add butter, and process until the mixture resembles coarse meal, 8 to 10 seconds.
With machine running, add ice water in a slow, steady stream through feed tube. Pulse until dough holds together without being wet or sticky; be careful not to process more than 30 seconds. To test, squeeze a small amount together: If it is crumbly, add more ice water, 1 tablespoon at a time.
Divide dough into two equal balls. Flatten each ball into a disc and wrap in plastic. Transfer to the refrigerator and chill at least 1 hour.
Preheat the oven to 375°. Roll the one dough ball out on a lightly floured surface into a 12" round, then fit into a 9" pie plate. Add the apple rhubarb filling and sprinkle the butter on top. Roll the remaining dough ball out on the lightly floured surface into a 10" round and lay on top of the filling. Cut off any excess with knife. Crimp the edges decoratively, and cut a few slits in the top of the pie. Bake for approximately 40 minutes or until bubble appear through the slits in the crust. Let cool before slicing.