For the Citrus Rosemary Salt: In food processor, process all the ingredients. Pulse until well blended. Set aside.
Preheat the oven to 425 degrees F.
For the Roasted Turkeys: Remove necks and giblets and put into a large saucepan. Add 2 onions, 2 carrots, 2 celery stalks and chicken stock. Bring to a boil over high heat and then reduce the heat and let simmer until reduced to about 2 cups. Strain and reserve; this is the turkey stock that you will use for the gravy.
Wash the turkeys, inside and out, and dry well. Coat each turkey, inside and out, with 1/4 cup of the olive oil. Season each turkey on the outside with a tablespoon or two of the Citrus Rosemary Salt, pressing it in to adhere. Place 4 rosemary sprigs and 2 lemon halves inside the cavity of each turkey.
Arrange 4 of the halved carrots and 4 celery stalks on each of 2 half-sheet pans (use baking sheets with a lip). Position each turkey on top of the carrots and celery so that the turkey does not rest directly on the bottom of the pan. Drizzle turkeys with remaining olive oil.
Roast until an instant-read thermometer (inserted deep into the thigh but away from the bone) reads 165 degrees F and juices in the thigh run clear when pierced with a fork, about 2 to 2 1/2 hours; begin checking at 2 hours. Remove from the pans and let rest for 15 minutes before carving. Reserve pan juices for gravy.
While turkeys are resting, make the gravy. In a medium saucepan, melt the butter and add the flour. Whisk over medium heat for 3 to 4 minutes, or until a smooth, blond-colored mixture is formed. Add the reserved turkey stock and pan juices and bring to a boil over high heat. Reduce the heat and let simmer until thickened and ready to serve. Season, to taste.