each other：互相,彼此之间.
强调“彼此”,用于两者之间.
例：Tom and David share their lunch with each other everyday.
one another：「一个……其他……」适用于三者.后常有「the third/the other」.
例：There are three pens on the desk.One is blue,another is yellow,the third(=the other) is red.
希望对你有所帮助.