Sibling Rivalry brothers David and Bob Kinkead can’t get enough of trying to outdo each other at their South End New American. They've added "Dueling Burgers" to their Sunday menu through Labor Day. Order the cheeseburger and you get both takes on this essential summer fare (think maxi-sliders). David's has Boursin cheese, pancetta, tomato and radish sprouts while Bob's is adorned with Anaheim chiles, Jack cheese and chipotle-lime mayo ($16; 617-338-5338).