If you think about this water boiler business real hard you'll come to the conclusion that this second boiler is just stupid over the multiple heat exchanger idea. This doesn't save any power over the plate exchanger it just takes a huge amount of space.
In theory this would allow for greater delta T because it has longer time to build up the heat. Adding enough plate exchangers would do roughly the same however.
Are you sure on that? With the extra water heater, you should be able to get the preheater to the same temperature as your water block exhaust temperature. With plate exchangers, you're still limited by the power of your loop. If you're running the hot water full out and pulling say 10lpm, you would need to dump ~700W of heat through the exchanger to raise the water 1 degree C. You would need a massive amount of plate exchangers and a lot of water in your cooling loop in order to raise the water temperature to close to the exhaust temperature. That sounds a lot more expensive than getting a free or very cheap water heater and some copper tubing, and much more likely to cause pressure issues on your hot water line.
Why?
700W/C is pretty insignificant when you have a 6KW heat source.
The reality is the amount of energy used in domestic water heating is a tiny fraction of the heat produced by mining. Thus getting ultra max efficiency is silly & expensive for no real tangible benefit.
I did the math already. A $50 flat plate heat exchange will dump about 90%+ of the energy I used each month to heat my hot water. Spending hundreds more to try and get the last 10% (most of which is simply unobtainable).
Your belief that you need a massive heat exchange or more water in the loop (?) is simply incorrect. The amount of water in the loop is immaterial. 1 gallon in the loop or 20,000 gallons in the loop. The amount of ENERGY in the loop is the same (it is the amount of energy used by the rigs).
I you're wrong on the last point. Consider a simple closed loop system where you have a pump, a block and a radiator. Your exhaust temperature will be a function of flow rate and power. Whether you have 2 ft of tubing between the GPUs and radiator or 20 ft, as long as you're not dissipating much heat through through the tubing the water between the blocks and radiator would be the same. You have 10x as much water in the tubing in the second case though, and thus 10x as much stored energy.
I don't see why that idea would be expensive though. It seems like something that could be done for $20, and perform as well or better than a large plate exchanger. The biggest downside is the floor space and low SAF.