The thing about "always two" is that it's an adversarial relationship. The goal of the apprentice is to replace the master. The goal of the master is to replace the apprentice.

Look at Anakin, Palpatine takes an interest in him early on. He begins to shape him, give him ideas, gain his trust, all while having Dooku as his apprentice. Once Dooku is useless he tells Anakin to kill him (Dew it!).

Dooku in that moment is shocked. What makes Palpatine the Master is that he was able to convince Dooku that he was the apprentice. Sure that may have been true at one point, but Dooku should have seen this moment coming. Especially since he replaced Maul.

One of the big things Maul comes to understand during the Clone Wars is that he is just a piece, a pawn. He may have been a Sith Lord, a Darth Maul, but he was never even close to becoming a Sith Master. We even see this play out with Savage. Maul thinks he can be a Master, he and Savage take on Palpatine and get destroyed.

Also look at Luke. Both Palpatine and Vader know about him (as of ROTJ). Both want to recruit him. If he could be brought to the dark side, then what? Vader says they could rule the galaxy as father and son. Maybe that means challenging Palpatine, someday. And Palpatine? That's a free new apprentice he gets to test out. And as we see in ROTJ he essentially does to Vader what he did to Dooku, his new apprentice is about to replace his old one.