MIX cherries, rhubarb, sugar, tapioca, vanilla, nutmeg, baking soda and red food colouring, if desired, in large bowl. Let stand 20 minutes. Heat oven to 450°F.
PREPARE recipe for double crust pie. Roll out dough for bottom crust. Place in 9-inch pie plate. Press to fit without stretching dough. Trim even with pie plate.
POUR filling into prepared pie shell. Roll out dough for top crust. Place onto filled pie. Trim 1/2-inch beyond edge. Fold top crust under bottom crust edge to seal. Crimp and flute edges. Cut slits in top crust or prick with fork to allow steam to escape.
BAKE 15 minutes. Reduce oven temperature to 350°F. Bake 40 to 45 minutes or until crust is golden. Cool on rack.