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.