If you can maintain the same temperature in the smoker box that you cooked at with the single chicken, your cook time won't change. Pre-heat the cabinet, heating a foil wrapped brick in your kitchen oven, then into the smoker cabinet, pour boiling H2O in your puck receiver, letting the chickens warm up on the counter for an hour, will help keep the cabinet temp. up if you were at max. temp. with the single bird. The second bird may extend the time some, but won't double it. You can also start early and FTC(foil/towel/cooler) them to keep them hot until serving time.