More RFIs from engineers and other informed readers.
A. Table 11.2.2.1 in the 2007 edition of NFPA 13 shows 850 gpm as the flow required for ordinary hazard occupancies, where you would need a residual pressure of 50 psi (see 11.2.2.5) at the highest sprinkler in order to use the pipe schedule tables for a system of any size, provided that the duration limit is met. In your case, the 50 psi is not available but each system would only be covering 4,000 square feet. NFPA 13 is concerned here with a single water source, and in the case of multiple risers, only one has to work properly at any time. The code is saying yes, it’s perfectly acceptable to base your design on each 4,000-sq.-ft. system based on the pipe schedule tables. You cannot use the pipe schedule tables for extra hazard occupancies or for situations where storage is expected to be piled higher than 12 feet. Two independent water supplies are not required.