The conversation still makes no sense, though. It is absolutely clear from what they are talking about that they are considering Luke as a powerful ally- a third member of their team. Why would they contact each other and gleefully discuss that they are now looking for what they consider to be each other's replacement?
The 'rule of two' fits in with a lot of things thematically but that ESB convo is where it falls short, continuity wise.
I dunno.... I kinda gives a nice twist to the matter. After all, all Sith masters should now that their apprentices will try to take over. It's no big secret. On the other hand if the Master wants to stay Master he'll have to replace a possibly rebelling apprentice. So they fact that they'd talk like this, could say something about a very cold and suspicious relationship and about an attitude of low esteem towards each other. It's pretty bold and arrogant to both play out the trump cards and pretend it's a normal card knowing that both players will not be fooled. It's like: cards are on the table, let the game begin and may the baddest win!
I dunno, I kinda like that.