I don't know of any command line to do this, but it not to say there isn't.
I do know of another way.
Go into device manager and change your system from uniprocessor to mulitprocessor.
MPS Uniprocessor-->MPS Multiprocessor; ACPI Uniprocessor-->ACPI Multiprocessor. Don't go from MPS to ACPI or vise versa.
You'll have to "update" yer driver to do this tho and reboot. All done!