Some people say u should buy the cpu's together as they work better 4 some reason-someone post reason if u know it.
If windows dosent boot after u installed 2nd cpu (like mine) u will have 2 reinstall. If it does boot go into device manager/computer and go to tree that pops out and click properties/driver/update driver. This will install dual processor drivers.
Technically, you should be able to install and run with just one CPU and then add the second CPU later. If Windows doesn't recognize it, then you have to update the HAL (very easy) and tell Windows that you now have 2 CPU's (also very easy.)