To a small saucepan, simmer the grapefruit juice until it reduces by half (to 1/4 cup), about 3-5 minutes. Set aside to cool slightly.
In a medium stainless steel bowl (note: you'll use this bowl as a makeshift double-boiler later). Add the sugar and beat until light and fluffy. Add the egg yolk, beat the mixture together, and then add the egg, beating again until it's well incorporated. Stir in the salt, and then in a few batches, add the grapefruit juice and lemon juice.
Rince out the small saucepan you used earlier, and fill it 1/3 of the way full of water. Bring to a simmer and place your stainless steel bowl of curd on top. Stir constantly, and heat the curd slowly enough that the sugar has time to dissolve. This step usually takes about ten minutes. Pull the curd from the head when it is just thick enough to coat your spoon, or, my preferred way, until a thermometer reads 166 F. Your curd will thicken as it cools.
If you feel like you've overheated it a bit, or you aren't using a thermometer, you can run it through a sieve to rid it of any lumps that might've cooked up. If you used a thermometer, then feel free to skip this step. Transfer the curd to a container and refrigerate, covered, for up to 1 week.