Secret Service has determined that it is in the country's best interest if these two individuals stay apart from one another as much as possible. If you watched the President's address to the nation, you would have noticed that Cheney wasn't sitting in the familiar "head senate" seat. He was at an undisclosed location as a security measure against a terrorist hit. About the only time the two men are together, during morning security briefings...
So far, his health hasn't been an issue since his last appearance in the hospital...