You mean when Vader and the Emperor were talking about bringing in young Skywalker, they both knew that he would have to kill one of them? That does not track.
Well according to Lucas thats the idea. Each being evil knows the other cannot be trusted, but they also need each other in the mean time. Each is banking on the fact that they will come out on top, and trying use the other's desire to do the same to their advantage.
or at the very least you could say that Vader does not know, but I don't think thats nessesary. Vader assumes he can get luke to kill the emperor, the emperor assumes he can get Luke to kill vader