Dual port 10Gbe performing at half speed

I have two servers running OpenSUSE both with  Intel dual port 82599EB 10-Gigabit SFI/SFP+ using the ixgbe drivers. And I'm not getting the bandwidth I expect with iperf. Both ports are connected directly to the the other two ports of the second server.

On one machine I run

iperf -s

On the other machine I run these two commands in separate terminals.

iperf -c 192.168.1.10 -t 20 -B 192.168.1.20

iperf -c 192.168.1.11 -t 20 -B 192.168.1.21

And I get

[  4]  0.0-20.0 sec  7.63 GBytes  3.28 Gbits/sec

[  5]  0.0-20.0 sec  14.7 GBytes  6.30 Gbits/sec

If I run only one port, I get

[  6]  0.0-20.0 sec  22.8 GBytes  9.80 Gbits/sec

Shouldn't I be able to expect roughly 10Gbe on each port simultaneously? Do I have the wrong hardware for that requirement? Or an invalid test?