Whisk 1/2 cup of the sugar, the egg yolks, lemon juice, lime juice, limoncello, and salt in a large metal bowl to blend. Set the bowl over a saucepan of simmering water (do not allow the bottom of the bowl to touch the water). Whisk the egg mixture until it is thick and creamy, and a thermometer inserted into the mixture registers 160 degrees F, about 5 minutes. Set the bowl of custard into another bowl of ice water to cool completely. Stir in the zest.