Light a grill. Cover 2 bricks with foil. Grill the chickens breast sides down, pressed with the bricks, over moderate heat until browned, 15 minutes. Flip and grill over low heat until the juices run clear when a thigh is pierced, about 25 minutes longer. Transfer the chickens to a carving board to rest for 10 minutes. Carve and serve, passing the chimichurri at the table.