I always wondered if this could be done. Now I can sleep easy knowing it can.
I wonder how efficient this would be. Do you think you would get the true power of all 4 kites out of this set up i.e 13+11+9+7 or whatever sizes they were, or do you think there would be diminishing returns due to the high and low pressure sides of the kite interfering with each other? Has anyone on here flown a set up like this?

Seen it done with foil kites but never inflatables. Wouldn't think wind flow would not be a problem at that spacing, back in the day "biplanes" worked and their wings were much closer together. From an aerodynamics point of view, the most efficient wing doesn't create turbulence but relies purely on the smooth flow of air over both surfaces, lift is created in the direction of the longer surface.

ive never been able to figure out the line set-up??wouldnt the front lines drag and wear on the LE of the kite below????

Bi-plane analagy is a good one - stacking kites (this is what this is called) would work fine as long as all kites can be controlled at the same time. I dont expect there is any synergy between stacks meaning 1 + 2 = 3 , simply you add the 'sail area' to make up the 'kite power'.