But his rule of two was created to enhance the power of the Sith.
Bane gets paranoid that his apprentice might just wait till he is frail and weak to challenge him. He doesn't think that Zannah would be more powerful then him because she could do that, no one would.
Deception is good, yes. Bane believes deception is a way of the dark side, yes. But he never wanted the final fight between the apprentice and master be a unfair tactic like killing your master in his sleep.
That is not the same thing. If that had happened, Zannah would have waited until Bane was frail and his health was failing. Plagueis health was fine, he was just caught sleeping in an accessible area, which is his failure. Bane was never caught off his guard like. He made a desparate attempt in his fight with Zannah with sorcery he had never done before and failed. Both Darths were in fine health, but they still lost. The way they lost doesn't matter. It's not like Plagueis was being held down. He should have subconsiously sensed Sidious through the Force and sensed his intentions.