Zeezrom springs the trap he had laid with the question about there being one God. He asks if the Son of God is the very Eternal Father. The two different titles certainly make it seem that there are two Gods. However, when Amulek responds, he simply affirms that they are the same, and then moves to the more important issue, which is that the very Eternal Father will come to earth to redeem his people.
Amulek’s simple answer makes it more difficult for modern readers who are used to a distinction between the Father and the Son, each as a member of the Godhead. That was not the Nephite understanding, however. It really was the Father, or Jehovah, who came to earth. For Amulek, Son of God and Eternal Father were just two names for the same being, but each name was appropriate for the location of that being. While in heaven, Jehovah is the Eternal Father. Coming to Earth, he is called Son of God.
This is not a metaphysical oneness, but rather the Nephite belief that it was that very God who came to earth. It is Jehovah with different titles in different realms. An explanation for this is given in the comments on 3 Nephi 1:14.