Combine some salt and sugar in a small bowl, and pour onto a small, shallow plate. Moisten the rims of 2 glasses with a piece of watermelon, and dip the moistened glass rims into the salt and sugar mixture; set aside.
Pour the watermelon juice, vodka, watermelon schnapps, simple syrup, and lime juice into a cocktail shaker over ice. Cover, and shake until the outside of the shaker has frosted. Strain into the prepared glasses.