Combine watermelon and strawberries.
Puree in batches in a blender, adding sour cream, milk and sugar to the last batch.
Pour into a 2-qt.container; mix well.
Cover and chill at least 3 hours.
To serve soup in cantaloupe bowls, cut cantaloupes in half; hollow out melon and seeds, leaving about a 1/2-in.shell.
Cut a decorative edge if desired.
Add soup; garnish with a strawberry if desired.