Open cans of mango and grapefruit. Separate fruit from juice, reserving both. In a small saucepan over medium heat, whisk together lemon zest, sugar, and cornstarch. Slowly add lemon, mango, and grapefruit juice, whisking constantly so cornstarch doesn’t create lumps. Bring to a boil. Whisk constantly for 3–4 minutes until sauce has thickened.