You're totally right. In such case those two complementing Forest object need to be driven by a "custom" Distribution Map (inverted on the second one) as in the attached sample scene.
Mm I see. But that require custom map painting which can take quite a while of time... + uving...
Luckily I found a good workflow to trim walls at the top and I am able to use large samples with it.
The top of the Hedge is standard XY surface with boundary detection and side is being trimmed by good guy vray :- ))
So far its working quite great! Got some holes tho that I need to fill with data or fine tune my settings but other than that my hedge is slowly getting there.
For now I'm using 20cm samples (3m height of hedge atm) I will try upping it to 40-50 cm samples and then I need to rework leaves alpha so it don't look like grass anymore heh.
Anyway thanks for your help !