Ask some help for the performance of Intel 82599 on Linux 4.1.15

Hi Jeff:

I use the Intel 82599 to run benchmark on my machine, the kernel version is 4.1.15, and I found that
when I run a single iperf by one port, the speed looks fine, it could run to 9.45Gb/s, it looks good. 
the command is: iperf -c 15.1.1.1 -t 100 -i 1

But when I run double iperf by two port from one single 82599 card, it looks strange, the speed could
only up the 6Gb/s + 6Gb/s = 12Gb/s, the total speed is not twice as much than one port, I try to
modify some param for tcp windows, it looks no effect, can you give me more suggestion?

Thanks
Ding