This explains why there is a seperate stock tank for the Calcium Nitrate concentrates.

This is from the "Tomato Handbook", MS State Extension Service. Use the link provided and note the link to the PDF version after the HTML document opens.

Go to page 11 of the PDF document
http://msucares.com/pubs/publications/p1828.pdf

The first and second paragrapsh under heading "Injectors" is interesting, but the third paragraph specifically addresses the question about "two nute tanks".

"Concentrate is held in small containers (e.g., 10 to 50 gallons). Two heads and two concentrate tanks (at a minimum) are needed: one for the calcium nitrate (tank B) and the other for all other nutrients (tank A). This is necessary so calcium does not combine chemically with phosphates or sulfates when the elements are in high concentration, especially under a high pH. The resulting compounds, calcium hydrogen phosphate or calcium sulfate, are hard precipitates and can clog the injector and irrigation system. However, once these elements are diluted, there is no problem. If the pH is higher than 5.8, it is advisable to use a third head to inject acid. This is needed to keep the pH in the 5.6 - 5.8 range (see pH section above)."

Here is a link that might be useful: Greenhouse Tomato Handbook