3/4 pound zucchini, cut into 1/4-inch-thick slices
3/4 pound yellow squash, cut into 1/4-inch-thick slices
2 teaspoons kosher salt, divided
1/2 cup thinly sliced sweet onion (such as Vidalia)
1 cup water
1 cup cider vinegar
1/4 cup maple syrup
1/4 to 1/2 teaspoon crushed red pepper