I think there is several things going on in this scene. Sidious has grown so arrogant that perhaps he thinks he should have two apprentices. This is the view I've had from childhood. (Stupid prequels for introducing the rule of two thing) The other is that Sidious is playing Luke and Vader off each other. Sort of a test for the two of them. The survivor gets to be the apprentice. He is just after the most powerful one, whoever it may be. Of course, this brings up the question, why replace Vader, who Sidious had powerful control over, with Luke, who he has almost none?