In a small bowl, combine grapefruit juice, lemon juice, salt, and agave or simple syrup.
Fill two highball glasses with ice. Pour half of grapefruit mixture into each glass. Fill to top with seltzer water. Stir well.
Notes
NOTE: Blue agave is a great natural sweetener for this drink because it dissolves into cold liquid. However, it has a very distinctive flavor that some folks don't like. If you aren't a fan, use flavorless simple syrup instead. To make, combine 1 cup water and 1 cup sugar in a saucepan over medium heat. Bring to a boil and heat until sugar is dissolved. Let cool and store in an airtight container in the fridge.