Put the biscuits in sandwich bag and use a rolling pin to gently bash them until small crumbs.
Tip the crushed biscuits into a bowl and pour in the melted butter. Stir together until well combined, then spoon the crumbs into the bottom of two small glasses and press down firmly.
Mix the lemon and lime juice, mascarpone and icing sugar together in a bowl until well combined.
Divide the mixture between the two glasses and chill in the fridge for two hours.