I have intel core2duo e6550, and i wonder is there some kind of command line which utilizes both cores for the cpu?

No, Nexuiz doesn't make use of dualcore, and doesn't need to... its CPU load is extremely low compared to the graphics card power it needs... so dualcore wouldn't accelerate it at all.

What DOES accelerate it is offloading OpenGL stuff to another thread (and another core). But that's done by default on all operating systems if the drivers support it (actually, on Windows and Linux, the drivers do this automatically, Nexuiz only has to enable it explicitly for OS X)

Thanks for clarfiying