I was just thinking in terms of storage. An average RO produces 75 - 100 GPD. Most Brute garbage cans are 35 - 50 gal. This gives you the ability to change up to 50 gallons of water. Why would you need a second 50 gallons right away? Even with a 200 gallon DT, you would normally only change 10 - 20% which would be up to 40 gallons. Then top it up and mix a new batch. You would still have enough for 25% volume on a 200 gallon tank. In this case it would be 50% on a 100 gal tank. Larger tanks would probably put you into a poly tank for volume, in which case you could get as big of a tank as you had floor space, and load capacity. A 200 gallon poly tank only takes up about 3 square feet and is 9' tall. I understand how the 2 tank system works, I'm just not sure why one would need the second tank. It's not like RO/DI needs to "breath" before mixing.
I have about 330 total system gallons. I do auto water changes of about 4G a day, and above and beyond that, there have been many times I wanted to do to a LARGE change, and believe me, wish I had a larger capacity. To you point, I have a 55" Poly tank, and no room for more than The size I have which is about 18" wide and 53" tall or so. There have been times, I have a need to do a larger change but I am half through the fill from the daily water changes so I don't have the full 55G either. For me, with the lack of space, I mounted 16G poly tank above that NSW tank that holds my DI water and feeds for ATO. I just have a straight pipe down with a shut off valve and a RODI line with a slitter running to each tank. Even still, once again as an example. I have half NSW and half RODI. I use the last 30G of NSW, and only have 5G of RODI to fill into the tank. Now I gotta wait. I have a 150GPD with a booster pump on it, and I still get annoyed. I only have so much time to allocate so when I get knee deep, I really just want to finish it, not have to come back hours later. You were looking for reasons why people would want them. I am just giving you some examples.
