I will get some pics later for you, have to get work done while the sun is shining.
The two in the unit now are in series. I will add an additional two in series to each other but parallel to the current two. This will divide the flow between the two membrane sets.
Flushing the membranes is simply a process to flood sap or permeate thru the system to remove sugar or contaminates on the filtration surface. I find that taking 2-4 minutes of allowing the unit to run at full concentrate flow will increase the flow of the membranes after production has slumped from filter plugging with sugars.
My wife says I can squeeze a fart out of a Buffalo on a nickel; but when it will make you $$$ then you need to spend a buck. I would suggest investing in flow meters and try those membranes you have. My Memtek has a inflow, membrane circulation, concentrate and permeate flow meters.
The plumbing is best done with copper lines where possible. I do use flexible tubing but I have to change them due to they are not rated for the pressures the unit achieves. Tube failure can cause serious injury and the wasting of sugar.
Go to run but will get back to you soon.
Ben