In a small bowl, mix together the coriander, smoked paprika, cumin. 1 tablespoon salt and 1 teaspoon ground black pepper.
Lightly coat the chickens with oil, then sprinkle generously with the salt and spice mixture.
Combine the tomatoes, garlic and onions in a large roasting pan and place the two chickens on top, breast-side up. Roast until an instant-read thermometer inserted in the thigh registers 160 degrees F, about 45 minutes (as the chicken rests, the temperature will rise to 165 degrees F).
Transfer the chicken to a large carving board to rest, then place the roasting pan on the stovetop and bring the tomato mixture to a boil. Boil for 1 minute and season to taste with salt and pepper. Puree with a hand blender or transfer to a blender to process until smooth. Reserve the sauce and serve the chicken.
After serving the chicken, pull any remaining meat from the bones and mix with the sauce. Freeze in small containers for up to 3 months.