To make the frozen yoghurt, in a small saucepan, bring the sugar and 1 1/4 cups (310ml) water to a simmer over medium heat, stirring until sugar dissolves. Cool completely. In a shallow baking dish, whisk the syrup, yoghurt, lemon juice, lime juice, lemon rind and 2 tsp of the lime rind. Freeze for 6 hours or until solid.
Use a fork to break the frozen yoghurt into small chunks. Process in a food processor, scraping sides as needed, until smooth. Transfer the frozen yoghurt to an airtight container. Freeze until ready to serve.
Position an oven rack in centre of oven. Preheat oven to 150°C (130°C fan-forced). Line a large rimmed baking tray with baking paper. In the bowl of a stand mixer fitted with a whisk attachment, beat egg whites until foamy. Gradually add the sugar, beating on medium-high speed for 7 mins or until firm glossy peaks form. Beat in vinegar, vanilla and salt. Sift cornflour over the meringue and gently fold to combine.
Scoop large spoonfuls of meringue onto lined tray to make 6 mounds. Use the back of a large metal spoon to make a slight indentation in the centre of each mound, pushing out meringue to create 8cm-diameter discs.
Place the meringues in oven and immediately reduce temperature to 105°C (95°C fan-forced). Bake for 1 1/4 hours or until meringues are crisp on outside but still have marshmallow-like centres and puff ever so slightly. Turn oven off and prop the door open with a wooden spoon. Leave meringues in oven for 30 mins to cool. Remove from oven and set aside to cool completely.
Meanwhile, in a large bowl, toss the combined peach and nectarine with the lemon juice and extra sugar to coat. Set aside, tossing occasionally, for 30 mins or until juices form.
Transfer pavlovas to serving bowls. Spoon over the peach mixture and juices. Top with a scoop of frozen yoghurt. Sprinkle with mint and lime rind. Serve immediately.