I have done Fire and HVAC since 81 and "My" interpretation is if the combined cfm of two or more AHU's or HP's into a room exceeds 2000 then then both or all get smokes. I have also seen Fan Powered VAV's that exceed 2000 get a DD with the exception of when multiple fan powered VAV's are feeding a large open area only the ones exceeding 2000 get them.
On a recent new construction project (in Tempe) the RTU HP's that all fed a big common area all had DD's in the return duct per the plans.
Depending on what the AHJ had for breakfast your mileage may vary.