In a large bowl or pot, combine the cold water, kosher salt, and brown sugar. Stir thoroughly until the salt and sugar have completely dissolved. The water should be cool to the touch; if not, add a bit of ice to ensure it is cold before adding the chicken.
Once the salt and sugar are dissolved, add the bay leaves, black peppercorns, rosemary, garlic, and any other herbs or spices you wish to include, such as lemon zest or thyme.
Place the two whole chickens in a large food-safe container or a resealable bag that can hold both the chickens and the brine. Pour the brine over the chickens, ensuring they are fully submerged. If they tend to float, you may need to place a plate or other weight on top to keep them submerged.
Cover the container or seal the bag, then refrigerate for at least 4 hours and up to 24 hours for optimal flavor infusion. Longer brining will result in more flavor and tenderness, but 4-6 hours will still provide good results if you’re short on time.
When ready to cook, take the chickens out of the brine. Rinse them under cold water to remove any excess salt, then gently pat them dry with paper towels.
Now the chickens are ready to be roasted, grilled, or prepared using your favorite cooking method. For oven roasting, preheat your oven to 375°F (190°C), and roast for about 2 hours, or until the internal temperature reaches 165°F (74°C) in the thickest part of the breast and thigh.