In a medium bowl, combine sweet and red onions; add ice water to cover. Soak for 30 minutes, changing ice water once. Drain onions well, and return to empty bowl. In a medium saucepan, bring vinegar, sugar, and salt to a boil, stirring until sugar is dissolved; pour over onions, and set aside to cool, tossing occasionally. When cool, stir in scallionos. Season with pepper.
To store, refrigerate in an airtight container, up to 5 days.