Mix the cucumbers, red onions, and 1 1/2 tablespoons of salt in a bowl. Pour them into a colander and suspend it over a bowl. Wrap the bowl and colander with plastic wrap, and place in the refrigerator to drain for at least 4 hours or overnight. Discard the liquid that collects in the bowl.
Pour the yogurt into a sieve lined with a paper towel and suspend it over another bowl. Wrap the bowl and sieve in plastic wrap and refrigerate for at least 4 hours or overnight. Discard the liquid that collects in the second bowl.
When the cucumbers are ready, roll them up in paper towels or a kitchen towel and press the towel lightly to remove most of the liquid. Place the cucumbers and yogurt in a large bowl with the sour cream, vinegar, dill, 2 teaspoons salt, and the pepper. Toss well and refrigerate for a few hours to allow the flavors to blend. Sprinkle with 1/2 teaspoon salt and 1/2 teaspoon pepper and serve chilled.
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