In a small saucepan, boil sugar and 1/2 cup water over high heat just until sugar dissolves. Stir in corn syrup and let cool.
In a bowl, combine syrup with juices. Freeze a metal 9-in. square pan. Freeze juice mixture in an ice cream maker according to manufacturer's directions. Transfer sorbetto to the cold pan, cover, and freeze until firm, at least 4 hours.
Let sorbetto soften in the refrigerator 20 minutes before serving in chilled bowls.
Make ahead: Freeze up to 1 week. Let soften at room temperature about 30 minutes, then break into chunks. Whirl in batches in a food processor until smooth, then scoop into chilled dishes. Serve immediately.
Note: Nutritional analysis is per serving.