Depends on the application. If you are dispensing at home, use one or two if the carbonation levels of the beer are different. They are designed to take a lowered pressure from a primary and lower the pressure further. They are not any more accurate than a primary nor are they applicable for high pressures from a primary. You can bank primaries together, but they are more expensive than using secondaries.
Scott Zuhse, Instructor
Micro Matic Dispense Institute