My NN actually thrive in the heat, which is a good thing since we haven't had a high below 100 since the first of June, lots of days pushing 110, low temps of 80. My NN will even sunbathe, but not for long. They also like being misted w/ water when it is hot, my roo will stand there to have his legs and feet hosed off. I recommend NN to all that I talk to when they complain about the heat. Since they are good producers, I consider them ideal chickens for Texas and the Southwest. They also handle winters well, despite the reduced feathering. They have enough down to be insulated when it is cold here. Basically our winters are below freezing at nights, and just above freezing during the day, and only 2 months of cold weather.
I have 5 NN hens and they don't lay double yolk eggs. Just large eggs, normally 5 or 6 a week from each girl.