To be perfectly honest, after putting a good amount of thought over the years into why some folks can't seem to "get" double unders, I had one theory that maybe that particular neurological synapse, or the connection of two neurons, needed for the coordination and execution of a double under just is not present in some people. Observing some folks putting effort into double unders, it just seemed like a connection so desired to be made, was not possible.
11am Tuesday we were doing a double under workout, and I looked over to see Rachel banging out some double unders! Smooth, coordinated, and rhythmic. You would have never known, unless someone told you, that for 7 years Rachel had been diligently practicing and putting effort into achieving the elusive double under. I am not sure how Rachel was able to get her double unders after seven years, but I write this for any of those double under seekers out there to not give up hope. That neurological connection could be nearer than you think.
With a partner, one person working at a time, 2 minutes ON - 1 minute OFF, 2 rounds:
Calorie row (10 pulls at a time, one rower)
Ball slam over ball
Medball sit ups
Prowler push (2nd round [if you have rope] pull prowler)