Stir together sugar, water and torn mint in a small saucepan. Bring to a simmer over low heat. Simmer 1 minute, then cool to room temperature, about 30 minutes. Strain syrup mixture through a fine sieve into a bowl, pushing on mint to extract as much liquid as possible. Discard mint leaves. Chill syrup until ready to use.
Stir together wine, orange juice, Triple Sec and mint simple syrup in a pitcher or large bowl. Add cantaloupe and watermelon and chill at least 30 minutes and up to 4 hours.
Pour or ladle over ice, if desired, into 8 glasses and garnish each with a mint leaf or sprig.