[gmx-users] cpu parallel

[ha salem]

Dear Users
I have 2 computer,each mashine has 1 core2quad cpu.
I have installed lamm 7,when I run mdrun -np4 on 1 machine cpu usage of 4 cores is over 90% and its s faster than single mode but when I run mdrun on 2 machines ,cpu usage of cores is about 30% and it doesnt get more speed
can you help me?
my network is gigabit and I connect 2 machines with gigbit cards and crooss cable
thank you