The meaning of the phrase “one on a side” is that the speaker and the neighbor repair the wall from opposite sides.
As a result of this, we can see that from the given excerpt, we can see that the narrator is that there is a kind of separation between him and his neighbor as they are on opposite sides.
While it is narrated that the neighbor is beyond the hill, there is the narrator on the other side of the wall and they are telling each other to stay where they are. Therefore, the phrase "one on a side" shows that both of them wants to repair the wall which holds them apart from each other.
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