Combine olive oil, white vinegar, red wine vinegar, sugar, 1 teaspoon salt and 1/2 teaspoon black pepper in a large bowl and whisk well. Partially peel cucumbers, leaving strips of peel remaining on cucumber. Slice cucumbers into 1/4 to 1/2-inch slices and add to vinegar mixture. Cut tomatoes into wedges and add to the bowl along with sliced onions. Toss well to coat. Season with salt and pepper.
Cover and refrigerate at least 1 hour before serving. Toss to coat before serving.