How to enable one virtual machine to use two processors?

I have a Virtual Server (R2 sp1) running on a box with two processor, each
processor has two cores. But I found one virtual machine can only use one
core. I wonder if MS support to let one virtual machine to use more than one
physical processor.

In addition, I want to know how to change the settings for “Maximum
Capacity�, on my system, I can only change the settings for� Relative Weight
Reserved Capacity�, �
(% of one CPU) Maximum Capacity� and “(% of one CPU) Reserved Capacity�.

Thanks!