I looked at the Unigine Valley benchmark demo today and saw something very interesting.
They render tree leaves with only one alpha masked polygon and then rotate that polygon so that it always faces the players viewing direction. That way, you always see it from the front side. The effect works so well, I did not notice it for a very long time, in fact you never notice it, unless you come from the render business and dissect everything. But even then it blend perfectly and natural to your eyes.
I made a quick sketch of what they are doing. No matter where you move the player, you will always see the alpha masked polygon like it is marked in red.
It should be much more effective that the current way with crossed polygons etc., if it is possible at all in UE4.