two of them 他们可能有很多人,其中的两个,没特指哪两个
比如：
They are altogether 8 people, and two of them have got mp3 players.
他们8个人,其中两个有MP3播放器.
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the two of them 他们可能有很多人,特指他们两个
They are altogether 8 people, and only the two of them have got mp3 players.
他们8个人,只有他们两个有MP3播放器.