Put the lemon and lime zest and juice, the sugar and butter into a heatproof bowl set over a pan of simmering water, making sure that the bottom of the bowl doesn't touch the water. Stir with a whisk every now and again until the butter has melted.
Lightly whisk the eggs with a fork, then whisk into the lemon mixture until well combined. Let the curd cook, stirring regularly, for 10 minutes or so, until it is thick, creamy and custard-like.
Remove from the heat and pour the curd through a sieve if you want to remove the zest. Stir occasionally as it cools then pour into sterilised jars and seal. It will keep for a couple of weeks in the refrigerator.