THE RULE OF THE TWO, "No more than two sith may be present at any time"
therefor when the emperor and vader are debating about luke being "turned", they must also be aware that one of them is going to kill the other one....(even though both of them openly suggest it at some point)
the only solution to this is that the emperor may not have told vader about this? or would he have known from his jedi teachings?
or would thay have disposed of tis rule....
(or the *annoying* fact that lucas kinda made it up after ESB)