In a medium-size saucepan, stir together water and corn syrup over medium heat.
Continue stirring until boiling (medium-high heat). Remove from heat.
Stir in powdered sugar with whisk, 1 cup at a time, until smooth and creamy without lumps.
Pour half of glaze into microwave-safe bowl.
Add chocolate to remaining half and stir until chocolate is melted and smooth.
Use an offset spatula to drizzle chocolate across center of cupcakes in a line. Fill in one side with chocolate glaze. Fill in other side with white glaze.
If glazes thicken, warm slightly in microwave or on stove-top.