Whisk the paprika, cornstarch, 1 tablespoon salt, the cumin, coriander and cayenne in a small bowl. Pat the chickens very dry with paper towels. Season inside the cavities with some of the spice mixture and rub the rest all over the skin. Refrigerate, uncovered, 15 to 30 minutes to dry out the skin.
About 30 minutes before roasting, bring the chickens to room temperature. Preheat the oven to 450 degrees F. Stuff the halved garlic heads into the chicken cavities. Tie the legs together with kitchen twine and tuck the wings under the body. Toss the baguette slices and garlic cloves with the olive oil in a large roasting pan; season with salt and arrange in a single layer. Arrange the chickens breast-side up on top of the baguette slices; leave a bit of space between the chickens.
Roast until the breasts and tops of the legs start browning, about 30 minutes. Rotate the pan and reduce the oven temperature to 350 degrees F. Continue roasting until the skin is crisp and a thermometer inserted into the thickest part of the thigh registers 165 degrees F, 35 to 50 more minutes.
Remove the chickens from the oven; let rest 15 minutes in the pan. Remove the twine and carve the chickens; serve with the roasted garlic and baguette.
The garlic cloves will soften during roasting, so you can squeeze them out of their skins onto the toasted bread.