Secret Conversation
There were two conversations that they were having. One that everyone can see and hear. No hidden codes. You can capture it on video.
The other was deep between their souls. The messages were passing between their eyes. No words, just ideas and emotions. Only they could feel it. Nobody else would suspect any form of communication other than the words they utter.
There's an unspoken rule that comes with this unspoken conversation; not to speak of it. They wouldn't need to of course, but if they were to do that, the magic of it all would disappear. Their souls would no longer be intertwined. For this connection only exists in silence.