1. Stir lemon sorbet, lime juice, and lime zest together in a bowl. Freeze until firm, 4 hours.
2. Melt butter and brown sugar together in a medium frying pan over medium-high heat. Add pineapple and cook until sauce thickens to a glaze, 8 to 10 minutes. Let cool.
3. Divide half the pineapple among 4 sundae glasses. Put 2 small scoops lemon-lime sorbet in each glass and top with half the toasted coconut, dividing evenly. Divide remaining pineapple among glasses and top each with 2 small scoops coconut sorbet. Pour in any pineapple syrup and top with remaining coconut.