The phrase "話し手も相手も共通に知っている場合" was translated in this board as "When both the speaker and listener know the subject"
I dont understand 2 things about this sentence. What's the use of ni in this phrase. It doesnt seem to match with any of the uses of "ni" I know. Isnt 話し手も相手も共通 the topic of the sentence and should be marked with "は". And why "the subject" isnt before the 知っている verb and it isnt marked with を ?