Peel the cucumbers and cut in half lengthwise. Using a small spoon scoop out the seeds and watery center from each half, forming a shallow groove down the center.
Using a sharp paring knife, slice the cucumbers crosswise as thinly as possible, ideally about 1/8-inch thick, or use a mandolin to get even, thin slices.
Place the cucumbers in a colander and sprinkle with the salt. Using clean hands, toss to combine the cucumbers with the salt and allow to stand, refrigerated, for at least 1 hour and up to 2 hours.
Using clean hands, squeeze handfuls of cucumber slices to release any excess liquid and transfer to a medium nonreactive bowl.
In another medium a glass bowl, combine white vinegar, cider vinegar, and sugar, and Crazy salt. Stir until the sugar is completely dissolved.
Pour the vinegar mixture over the cucumbers, adding onion slices, the chopped chives, and red pepper flakes (if using). Stir to combine, serve, or refrigerate up to 12 hours or overnight and serve cold, with a slotted spoon on the side.