RARE AIR: Industry veterans Winston and David Simone have just made a literally breathtaking career move, as their management client Nik Wallenda pulled the biggest audience ever for a live program on the Discovery Channel with his high-wire walk across the gaping expanse of the Grand Canyon on June 24. The real-time nail biter was the highest-rated show on any channel that evening. The brothers executive produced with Shelley Ross. They also manage viral sensations turned newly minted stars The Piano Guys, who've amassed 1.7 million YouTube subscribers and 250 million views. (7/8a)