Best flight time of a 3dr X8

I am using a 3dr X8 model 2013, with 2 batteries in parallel (each one: 4200mAh, 3Cells). I get a flight time of 7 minutes (voltage drops to 11.1 V). Do I have to use other batteries to get more flight time? Or, is there any think I have to do to increase the flight time?
N.B. I always have an additional payload of 350 grams.