Stir sugar and water in a small saucepan over medium-high heat until sugar dissolves, bring to the boil, then cool.
Place pineapple, mint, lime juice, coconut water and cooled sugar syrup in a high-speed blender and blend until combined (consistency should be like soft serve).
Pour into six large popsicle moulds, filling to about 2cm below the rims, and freeze until surface is set (1½-2 hours).
Meanwhile, whisk Greek yoghurt with caster sugar and water until combined. Pour on top of the frozen pineapple mixture to just below rims. Insert popsicle sticks and freeze overnight.
Dip moulds in hot water to remove popsicles, and keep frozen on a tray lined with baking paper until ready to serve.