In a blender, juice the watermelon and rhubarb together (you may have to do this in two separate batches). Immediately strain through a fine-mesh sieve, pushing out all the juice with a spoon. You will have about a half cup of fibrous pulp leftover.

Mix the watermelon-rhubarb juice with the tequila, triple sec, simple syrup, and lime juice in a pitcher. Salt the rims of 8 glasses, and divide the margaritas between each glass. Garnish with extra watermelon, lime slices, and rhubarb.