I tried this about three weeks ago and I just couldn't get her to focus on which bird to pick up, not run between, switch birds or just be confused. Gave it some time, worked on longer single marks and bumpers falling where she couldn't see them. She has been doing well on those so I decided to revisit double marks. Today she did well. Started off short, fifteen or twenty feet, and worked out to about fifty feet. Had a couple of corrections and call offs at first but she quickly got the jist of it and did well.