Hi Scooty, your system is coming along nicely, the green water tank is a great idea to minimize algae growth, mine is translucent, I hope that doesn't lead to that problem..
I've got a spectrapure 100 gpd unit, this time of year the cold water is warm enough that I get full production, in the winter the water temp drops and my production drops substantially. Your dual set up sounds like a great idea, sounds better than the pressure pump concept, I imagine the second RO unit costs less than a pump, and you are not adding an extra mechanical device to your system, simpler is always the way to go if you get the same results, isn't it...
Presently all my reject water is used in our gardens, typically we get little rain in the summer months. Different story the rest of the year here, the Seattle area is not know for it's winter droughts, most of the reject will go down the drain, very wasteful, especially with the volumes I'll be dealing with....
I recently decided to use two separate RO units, the run from the garage where the mixing vat is to the aquarium site would be quite long and would affect the efficiency of the RO production if I used it for tank top off, and using this same unit for water changes would add another level of complexity, etc. one way or another, things will be cleaner and simpler to have 2 units, one close to the tank for topping, another in the garage for the water changes.
Taking nine months to design and plan my system has worked out well, I have enjoyed this part of the process, in the past I have always just "gone for it", and then spent the next couple of years fixing/improving things, money and time could have been saved with better planning and execution. I'm with you, tinkering is great fun, I'm to the point where I'm burnt out on planning, I want to get my hands on everything and start playing with it, it's like when I was a kid, the closer we got to Xmas the more time slowed down, help me, I'm caught in a time warp....
I'm really excited about the OM's and CL setup, I have ideas about what I want to accomplish with the flow within my tank, It is gonna be fascinating to see where that goes when variables like the live rock are introduced, a tweaking we will go....