He wants a joint sitting of the Congress because this speech will have one purpose, and one purpose only; to lock in the last bastion of support that he has - the AIPAC-frightened Cowards of Congress - so that they can act as his bulwark against The Rest Of The World. Nothing more. No less. And, let's face it, he has a point i.e. they are the only people left on this planet who even *pretend* that they are still listening to his duplicitous nonsense.
from the article: Netanyahu: Binational state would be disastrous for Israel