Hi CJ
As we know green dot is the L/D max point on total drag curve.
That is the the point where the drag is minimum for a given weight. For a given weight the total drag is calculated as a sum of parasit drag (=profiile drag) and induced drag. The Parasit drag curve shape is different for dual and single engine case. In the single engine case, the parasit drag increase with increasing CAS, is more rapid and sharp. Eventually this moves total drag curve to left relative to dual engine. The result is L/Dmax point (green dot speed) corresponds to less speed than dual engine.