Why Two regression Lines

Once regression line  cannot  minimize the sum  of square of deviations  for both  the x and Y series  unless the relationship  between  them  indicate perfect  positive or negative correlation. In case of perfect correlations one regression  lien is enough  because X and Y series have the  same type  of  deviations. Ordinarily in social sciences perfect correlation is  very  rarely   found. For  this  reason  one  regression line  minimizes  the sum  of the  squares of deviations  of the X series  and the  other  regression  line  takes  care  of the deviations y series.