If two people talk at the same time and at the same level, it is possible to hear only one if we decide to focus on, then we do not hear the second person at all. But the brain will still hear perfectly both persons. The words can still be recognised by incouscious brains, not the complew sentences wich involve conscious levels to be undrerstood.
But recognising words are suficient here to brainwash
It is necessary to associate the keywords before, for example during an hypnosis session.
When our subject is speaking to someone else, for example in a restaurant, we can sit at a table near from him, then when they are talking together we can phone and say sometimes the keywords in our conversation.
This will make the subject feeling in relation to what he have associate with the keyword, also this will modify his conversation direction without he can be aware why