I never got the impression that there was any communication between Denethor and Saruman using the Palantirs. Each seemed to use their seeing stones to gather information on what was going on in the world for their own personal purposes. Denethor was said to be a wise and far seeing man but no one, not even Gandolf, knew of the stone until the pyre.

I believe that Sauron broke Denethor by controling the stone without Denethor's knowledge. Showing him distorted images of his strengh much like a news report where there is a crowd of people with only 3 or 4 fighting and the others just standing around but the camera focuses on those fighting, therefor making it look like everyone is. I believe Denethor never knew that he was being deceived. If he had suspected that Sauron was using him he would have stopped using the stone. Denethor was a strong willed and wise man. Only a fool would have kept using the stone knowing that the enemy controlled it. He would never have "spoken" with Sauron as Saruman apparently did. Therefore, Sauron would not have been able to learn from Denethor that Frodo and the ring were heading into his own land. Sauron still assumed that Saruman had captured it and when the Nazgul were dispatched to fetch it they would have seen the distruction of Isengard. This coupled with Aragorn revealing himself would have made Sauron assume that the ring was captured by Aragorn and headed towards Gondor where it would be used against him. This made Sauron panic and he struck Gondor before he was truely ready.

Tina