Installation of two range boiler tanks

bighat

I have a flame view wood burning cook stove with a water back to heat water. The 40 gallon tanks sits about 2 to 3 feet from the stove and thermosyphons the hot water from the stove. The system pressure and is unvented with TPR valves. The system produces to much hot water at night and during long fires. I would like to add and another tank but I'm not sure how to plumb it. I've included a paint brush picture of my stove and setup. You'll notice the blue rectangle to the left, this is the only location I can add another tank. Look forward to some good ideas, thanks