How many days out does it take to plan a nationally televised debate. I see no specific date or even a station anouncing it would pick it up. We are nearing the end, and do not have much time left. I think it's just a publicity stunt and won't happen.
BTW : I think sanders and trump are so far apart in terms of political theology and even how they view the world that the debate would be very awkward. It would be like an exclusive football fan arguing with an exclusive basketball fan about soccer.