Will go back up and read up on what Parjarita said in a minute, but I wanted to say, are we talking long term or short term separation? I am of the opinion that short term separation is fine (taking bird to another room for taming/training such as recall practice), but long-term separation is cruel. My cockatiel and parakeet ignored each other completely but one day I placed the keet's cage just outside the patio screen (can't remember why) and my cockatiel went crazy to the point of climbing up the screen while calling to her. They were inseparable after that.
I do think it causes the birds stress and if they are now tame, you should reunite them. While it may be true that they become tamer faster, isolating a bird from their companion is cruel.
I apologize if anything I said is repetitive

Edit: I just went back over all the posts and realized we are speaking long-term. OP, I really think you should put your birds together. While they may be more obedient towards you, their mental health is compromised. When you put them back together, their behavior will become altered, yes, but seeing as you already have such a large flock, I don't think it would hurt you too much. It would benefit the birds, while separating them causes emotional stress.Also, you said you don't see any signs of stress, but birds are animals which are made to hide their signs of illness or discontent.
Forget separating them, a bird that is alone is much more discontent as opposed to a bird living with another. I truly believe that it is in their best benefit to put them back together.
Props to you to living with all the noise though. We just got a ringneck and a quaker and their flock calls give me headaches.
Parjarita, I think people tend towards comparing them to children as to us our parrots' behavior reminds us of human toddlers. But you are correct, they are sexually mature individuals, a point which, while easy to forget, would do us all much good if we remembered it.