Read the excerpt from "mending wall."

i let my neighbour know beyond the hill;
and on a day we meet to walk the line
and set the wall between us once again.
we keep the wall between us as we go.
to each the boulders that have fallen to each.
and some are loaves and some so nearly balls
we have to use a spell to make them balance:
"stay where you are until our backs are turned! "
we wear our fingers rough with handling them.
oh, just another kind of out-door game,
one on a side. it comes to little more:
there where it is we do not need the wall:
he is all pine and i am apple orchard.

what does the phrase “one on a side” mean?