John told him that the two of them actually met many years ago, but it wasn't until recently that they found a soul mate in each other.?

> it wasn't until recently that 用法？
它的意思是="...直到最近...才..."

it wasn't until recently that they found a soul mate in each other.
= 直到最近 他們2人 才 在對方 的心靈中 找到 夥伴.


> 爲什麽要用"in" each other?
因為 "心靈伴侶"是抽象的,內心的 所以是在 心靈"之中"找到的. 所以用"in".

寫作除了表達"事實外"還要加上些時空的形容以表達事情(這裏說"感情")的前因後果或時間關係。

it wasn't until recently是一種插入句來加強that後所說明的結果跟前句的時空關係。前句說明他們已認識了多年，假如沒有這附加的時間點，後句表示他們"已經找到大家成為"靈伴"=情侶"便難以解構他們在那多年中何時開始"一起"/"結合"。

in each other用IN是因為表達"心靈"/"感情"/"關係"時的each other相互關係並不是"外在"而是"內在"的。
當然另外常見的類似寫法是they are fond of each other.意思是大家都喜歡大家(互相傾慕)，但層次上這是較為"外在"的關係，而soul mate則是內在的關係。

一個簡單的直述句
I met John many years ago in Taipei.
這個句子有主詞、受詞、時間、地點，如要特別強調某一要素，可改寫成
It was I that met John many years ago in Taipei.
It was John that I met many years ago in Taipei.
It was many years ago that I met John in Taipei.
It was in Taipei that I met John many years ago.

原句 ...but 之後是一子句
..., but it wasn't until recently that they found a soul mate in each other.
= ..., but they didn't find a soul mate in each other until recently.

each other 是 "相互之間，in 的使用就如Prisoner所闡述，如 I found myself in you. 句中 "in"的使用，也如Simon所說，除了in都不搭，少了in就不對味。

四個人：John, him, the two of them
actually actually met many years ago ：實際很多年以前便以碰過面
but it wasn't until recently that：過去沒有，但現在已發生
they found a soul mate in each other：他們兩位各自找到對方為自己的心靈伙伴
each other : 如字面意思即彼此。至於為何一定要用in不能用at 或 on，有一個說法還蠻可参考的，即at用於點，例如I’ll meet you at school – 盡管學校很大，您還是可把他視為一個碰面點。on用於線或面，如on the road，因road 符合是一條線。in用於實體空間或抽象空間情況，如本句中所用之 in each other或 there is an elephant in the room – 雖然房間很小，但這個大家不想講的問題- elephant，還是存在於室內 (不適用點，也不適用面，而是室內，實際上指的是室內所有人這個抽像空間)