To prepare chickens, cut them on each side of the backbone using poultry shears or kitchen scissors. Rinse the chickens and dry with paper towels. Open out on a board, skin side up, and press down with the heel of your hand on the top of each breast to break the breastbone and to flatten it. Cut deep slashed diagonally in each breast and on the legs. Using two long metal skewers for each chicken, push skewers from the tip of each breast through to the underside of the legs, which should be spread outwards so that the thickness of the chicken is as even as possible.
Put saffron in a mortar with the salt and pound with a pestle to pulverise the threads. Add the garlic and pound to a paste. Work in the paprika, cayenne pepper, cumin, black pepper, lemon juice and olive oil. Rub the spice mix into the chickens, rubbing it into the slashes. Cover. Marinate in the refrigerator for at least 2 hours, or overnight. Bring to room temperature 1 hour before cooking.
Prepare a charcoal fire or preheat the barbecue and place the chickens on the grill, skin side up. Cook over medium heat for 20 minutes, continually turning the chicken as it cooks and brushing with any remaining marinade. The chicken is cooked if the juices run clear when the thigh is pierced.
Cooking time can be shortened on a barbecue if a roasting tin is inverted over the chickens to act as a mini oven - reduce the heat to low to prevent burning. Transfer the chickens to a platter, remove the skewers, cover with a foil tent and leave to rest for 5 minutes before cutting in half to serve.
Quarter the lemons and dip the cut surfaces in the sifted icing sugar. Place on the barbecue hotplate. Cook briefly on the cut surfaces until golden and caramelised. Serve the chickens with the lemon quarters and watercress.