Place cucumbers, onions, pickling salt, and water in a large, non-reactive bowl. Cover and allow cucumbers to soak for 2 hours. Drain the water from the onions and cucumbers through a colander and rinse well for 5 minutes. Drain well and set aside.
Combine the vinegars, sugar, mustard seeds, turmeric, cloves, garlic, and peppers in a medium saucepan over high heat. Bring to a boil, reduce heat to medium and add the cucumbers and onions. Bring to a simmer and remove the saucepan from the heat.
Fill each of the hot sterilized pint-size preserving jars with the pickle mixture, dividing them evenly, and enough of the liquid to come within 1/2-inch of the top. Add 1/2 teaspoon of Natural Pickle Crisp to each jar, if desired. With a clean damp towel, wipe the rim and fit with a hot lid. Screw on the metal ring just until the point of resistance is met. Process the jars in a hot-water bath for 15 minutes.
Using tongs, remove the jars, place on a towel, and let cool. Test the seals by allowing the jars to stand at room temperature overnight or until the lids pop. Tighten the rings and store in a cool dry place. Let the pickles age for at least 2 weeks before using.