>>Even though I expressed it in "wave angle" terms I was actually overlaying the two EZNEC elevation traces and comparing them across all low elevation angles from 1 degree up. If we are seeing different results I need to re-check my modelling
I just ran some models using simple 40m dipoles to keep things simple and looked at wave angles from 1 to 5 degrees.
One at 180ft plus one at 90ft was a significant improvement over one at 180ft.
I then dropped them to 50ft and 25ft. The single at 50ft was significantly better than the stacked pair.
Around about 120ft + 60ft was where the stacking began to show advantage.
So the message seems to be that it's height dependent.