For the sauce: In a bowl whisk together the ketchup, BBQ sauce, sugar, vinegar, Worcestershire sauce, mustard and BBQ rub seasoning. Then pour the sauce in a thick-bottomed saucepan over low heat. Cook to blend the flavors, 10 to 15 minutes. Once warmed, remove from the heat and hold in a warm spot.
For the chicken: Preheat the oven to 300 degrees F. Drizzle both sides of each chicken with the oil. Next, mix the salt, pepper and cumin in a bowl, and then sprinkle over both sides of each chicken. Place the chicken, breast-side up, on a flat cookie sheet and cook until the internal temperature reaches 165 degrees F, 30 to 35 minutes.
When fully cooked, remove the chickens and lightly glaze the skin with the BBQ sauce. Return the chickens to the oven, allowing the BBQ sauce to glaze the birds, 10 to 12 minutes.