Mix sugar and cream of tartar in medium saucepan. Gradually stir in water. Bring to boil on high heat; simmer on low heat 5 min. or until mixture is reduced to 1-1/2 cups. Stir in zest and juice; cool 30 min.
Heat oven to 425°F. Roll out 1 crust on lightly floured surface to 11-inch circle; place in 9-inch pie plate. Place cracker crumbs in crust. Pour sugar syrup over crumbs; top with butter and cinnamon.
Roll out remaining crust to 10-inch circle; place over pie. Seal and flute edge. Cut several slits in top crust to permit steam to escape.
Bake 30 to 35 min. or until golden brown. Cool.