Mix vinegars, water, sugar and salt in a bowl until the crystals dissolve.
Score the skins of the cucumbers with the tines of a fork all around the vegetables. Slice the cucumbers thinly. Slice the onion thinly and cut the slices in half.
Add the cucumbers and onions to the liquid mix and stir them to coat. Let salad brine for an hour or two at room temperature and then refrigerate until ready to eat.