dirkclod I use FPVLR 2.4ghz lhcp half sphere helix 9 dbi antennas on my extender. They have a nice wide radiation pattern. 76 degrees I think. Really helps keep wifi signal in contact with the bird. Yes I use 2 of them. Never have any problems with close reception.
My flying is generally from 0 to 3000ft but at very low altitude. Less then 100ft. Not many obstructions in the desert. If your looking for long distance flights only, you may want to think about higher dbi wifi antenna.
I added two 2.4ghz lhcp pinwheel antenna to the bird to maintain consistent signal while exploring but don't think they added any significant range increase.