For the simple syrup: Combine the water and sugar in a small saucepan with the chopped rhubarb. Bring to a rolling boil and let cook for 10 minutes until the sugar is dissolved the the rhubarb has released its color and flavor. Remove from stove and set aside to cool.
For the rhubarb margarita: Combine the freshly squeeze orange and lime juice with tequila, cointreau, club soda and simple syrup. Shake in a cocktail shaker for 10 seconds and then serve.
Makes 2 very large rhubarb margaritas