Dry chickens inside and out with paper towels, then carefully loosen breast and thigh skin with a butter knife and your fingers. Rub chickens inside and out and under skin with salt (about 1 tbsp. per chicken). Chill chickens, uncovered, on a rack set in a rimmed baking pan, 2 days. Let chickens come to room temperature before roasting, about 1 hour. If at all moist, dry with paper towels. Put 1 large or 2 smaller cast-iron skillets, or a heavy roasting pan with a flat rack inside, in oven and preheat to 450° (475° for roasting pan).
Meanwhile, toast peppercorns: Discard any leaves or loose twigs from peppercorns. In a frying pan (not nonstick) over medium-low heat, toast peppercorns, shaking and stirring occasionally, until fragrant and slightly darker, 4 to 6 minutes (if they start smoking, lower the heat). Pour into a bowl and let cool completely, then grind to a coarse powder in a spice or coffee grinder.
Sprinkle chickens all over, inside, and under skin with most of Sichuan pepper (about 1 1/2 tbsp. per chicken).
Place chickens breast side down in skillet or pan. The chickens should have plenty of room around them for hot air to circulate. Roast chickens until skins begin to brown, about 20 minutes. Using 2 spatulas, carefully turn chickens over and sprinkle with remaining Sichuan pepper. Roast another 20 minutes.
Reduce heat to 350° if using cast iron (400° for roasting pan). Roast until an instant-read thermometer inserted into thigh joint registers 165°, 10 to 20 minutes. Remove from oven and let rest 10 minutes.
Tip juices from chickens into skillet or pan, then transfer chickens to a cutting board. With a chef’s knife or poultry shears, cut out back and trim wing tips (save for stock if you like). Cut each chicken into 8 to 10 bone-in pieces (cut breasts in half if they’re big). Return cut-up chickens to skillet or roasting pan to serve, and top with chicharrónes and cilantro. Serve remaining chicharrónes on the side.
*Find Sichuan peppercorns at well-stocked grocery stores, specialty markets, and online. Find chicharrónes at Mexican markets, some grocery stores, and online.