How to increase Air flow while operating with two radial blowers?

I have got two radial blowers operating in series with suction of each blower open to 100%. The operating capacity of each blower is 2900 m3/hr at 5400 rpm with 300 mbar differential pressure. While operating both the blowers in series, I find the total pressure increase across both the blowers to be 390 mbar against expected of (300 + 300), and flow increased from only 2000 m3/hr to 2500 m3/hr instead of doubling.