The figure of speech used in the lines 9-10 is:

Lines 9-10 read: 'And on a day we meet to walk the line / And set the wall between us once again.' The wall is being treated as something with agency that exists 'between us,' giving it human-like presence. However, considering the overall context where stones are addressed directly ('Stay where you are until our backs are turned'), the figure of speech of giving non-human objects human qualities is personification.