Go to Start -> Control Panel -> Administritative Tools -> Microsoft Dynamics AX 2009 Server Configuration
in the performance tab you can setup the AOS to use more than one CPU. After you change the default settings you'll have to restart the AOS service.
Just in case the AOS is frequently crashing after you change processor affinity the best option would be to install a second AOS on the same machine and setup a single CPU per AOS.
Regards,
QK