Preheat the oven to 375°. Spray a large non-stick baking sheet with oil. In a large bowl, whisk the mayonnaise and yogurt with 1 tablespoon of the salt and 1/2 teaspoon each of the cayenne and black pepper. In a large resealable plastic bag, mix the polenta with the flour, the remaining tablespoon salt and the remaining 1/2 teaspoon cayenne. Add the chicken to the yogurt mixture and turn to coat completely. Transfer the chicken to the plastic bag, a few pieces at a time; seal and shake to coat.