Assigned 1CPU + 2cores don't works

Where are you looking when you say you see only 1 cpu?
Which Windows is it?

If the web interface shows a 1-socket 2-core cpu
AND
If Windows shows 1-socket 1-core cpu,

My first thought would be that it's XP Home edition, or an equivalent which does not support SMP (multi-core/socket), and thus will not recognize any more than 1 core/socket.

In XP, only the Pro edition is able to do SMP.
Vista Home Premium & Vista Ultimate can do SMP.
I don't know about 98, ME or 2000.
 
Microsoft says it should work:

"The Windows Server 2003 family supports single or multiple CPUs that conform to the symmetric multiprocessing (SMP) standard."

I don't know what else to tell you, I'm stumped too.
 
I know if I uninstall a core (real- not QEMU) via Device Manager in Vista, after reboot it shows 3 active and the 4th as an unknown device until the Update Driver sequence is done installing it.

Sometimes I'll ruin my network so bad that I have to uninstall the NIC via Device Manager & let Windows reinstall it for it to work again.

It's a shot in the dark, but maybe the same will work for your CPU.