Place cucumbers in a large bowl and toss with salt; cover and refrigerate for 1 hour. Transfer to a colander and rinse under cold water to remove the salt. Drain well and return to bowl; add sliced onion.
In a medium saucepan combine sugars, vinegars and remaining ingredients. Bring to a simmer over medium heat, stirring until sugar is dissolved. Pour hot pickling liquid over cucumbers and onions. Allow to sit at room temperature for 1 hour.
Transfer pickles to covered jars and refrigerate for 24 hours. Store refrigerated for up to two weeks.