Preheat the oven to 400 degrees.Remove giblets from the cavities of the chickens.Rinse inside and out with cold water, then dry thoroughly with paper towels.Place them breast-up in a large roasting pan (or two smaller pans, if you don’t have a big one).In a small bowl, combine the softened butter, olive oil, garlic, salt, pepper, and herbs. I run my fingers between the skin and the meat of the chicken to loosen it, then rub half of the butter mixture under the chicken skins and half over the top of the chickens — the butter on the meat itself, locked under the skin, keeps the meat really juicy and flavors it nicely.Rubbing herb butter over the skins browns them well and makes the skin nice and crisp.Stuff the cavities of the chickens with the lemons, oranges, and onions.Roast the chickens at 400 degrees for about an hour and a half, depending on the size of the birds. The way I figure time for this is to take the average weight of one of the chickens (say, 4.5 lbs) and calculate 15 minutes per pound (that would be about 67 minutes for a 4.5 lb chicken). Then I round up — making it about an hour and 10 minutes for two 4.5 lb chickens.After cooking, let the chickens rest on the countertop, covered with foil, for about 15 minutes prior to carving.