Staying on topic, I'd say it looks as if you can only enter each Silent Realm one time. The purpose of going to the Silent Realm is to collect the Tears of the guardian of that area, similar to collecting Tears of Light in TP. That being said, once collecting the Tears is completed, I'd say the Silent Realm becomes pretty useless and probably inaccessible for the rest of the game. I'm pretty much making this assumption based on the fact that after collecting the Tears of Light in TP, that area could not be visited covered in Twilight anymore.
Of course, I could be completely wrong.
