Trim chickens of excess fat, rinse inside and out, and pat dry.
In a small bowl, combine paprika, chili powder, oregano, salt, black and cayenne peppers, garlic powder and brown sugar; mix well. Rub about a teaspoon of the mixture on the inside of each chicken. Rub remaining mixture over the surface of the chickens.
Open beers and pour off (or drink) about half of each beer. Stuff the diced onion and garlic into the cans. Ease the chickens over the beer cans so that they remain upright.
Place chickens and beer cans on grill. Close lid, leaving vents open. Grill for about 2 hours (if you use a gas grill use lowest setting), or until legs and wings wiggle easily.