In a cocktail shaker combine the first five ingredients (through lemon juice); shake to combine. Add ice cubes; cover and shake until very cold. Strain into two chilled martini glasses. If desired, add a grapefruit wedge to each serving.
To make simple syrup, in a small saucepan, combine equal parts water and sugar; cook and stir over medium heat until sugar is dissolved. Cool and store in the refrigerator until needed.