Multiple CPU licensing question.

I messed up and bought a default license. And then scratched my head trying to figure out why I couldn't assign a license to my ESX box. Turns out I didn't read closely enough and I need *2* licenses for my dual Xeon system. I either have to try to get a refund or double up, but in the meantime I was hoping someone could answer this:

Can I just buy another single CPU license and add this in the license section for the required total of two available licenses? Or do I have to contact sales and get a new single serial number that contains 2 licenses within it.

I hope that makes sense.

Many thanks.