Whisk together egg yolks and 2 tablespoons water in a heatproof bowl until doubled in size and pale in color.  Set the bowl over a double boiler until mixture doubles in size and begins to thicken, about 1 minute. Add half of citrus juices and whisk constantly, until thickened slightly, 1 to 2 minutes more.

Remove bowl from heat and add butter in a slow, steady stream while whisking vigorously until combined. Serve warm. Sauce can be stored in the refrigerator in an airtight container for up to 1 day.

Gently warm over a double boiler before serving.