Preheat the oven to 400°. Arrange one crust in the bottom of a pie pan, pressing the bottom and sides of the crust. Set aside.
In a large bowl combine the rhubarb, strawberries, brown sugar, white sugar, arrowroot powder, salt, lemon zest and lemon juice. Toss to combine. Transfer the berries and rhubarb to the pie shell, leaving any excess liquid in the bottom of the bowl and spread evenly into the crust.
Cut the remaining crust into 1" strips. The strips on the ends should be reserved for the end pieces on the pie and the center strips are used for the center pieces. Weave a lattice crust, then tuck any loose ends underneath the overhang of the bottom crust and pinch the edges into a decorative border. Brush the crust with the heavy cream and sprinkle with demerara sugar.
Place the pie on a baking sheet covered with aluminum foil to catch any spills. Bake for 20 minutes at 400°, then turn the oven down to 350° and bake for an additional 25-30 minutes.
Remove from oven and let cool to room temperature. Serve.