Bring water, sugar and salt to boiling in medium-size saucepan. Remove from heat.
Peel and seed cantaloupe; cut into chunks. Place in food processor or, working in batches, in blender. Whirl to puree. Pour into 9 x 9 x 2-inch metal baking pan.
Repeat with honeydew, pureeing; pour into second 9 x 9 x 2-inch square metal baking pan.
Add 1 cup of the water mixture and all of lime rind and juice to pan of pureed cantaloupe. Pour remaining water mixture into pureed honeydew. Cover each pan; freeze until partially frozen, about 3 hours.
Spoon cantaloupe mixture into food processor or, working in batches, into blender. Whirl until smooth, but do not over process and melt sorbet. Return mixture to pan. Cover; freeze until firm, 3 to 4 hours. Repeat the whole process with honeydew mixture.
Remove sorbets from freezer about 15 minutes before serving for easier scooping. Garnish with mint sprigs if desired. Makes 16 servings.