Can I use my single user license on 2 of my computers without purchasing another one?

No the license is per WhatsUp installation.

What is the difference between the site and the global license?

The site license grants you the right to use the software to all the WhatsUp installation over a single site .

The global license is the same thing for multiple offices and it can be used worldwide if you have multiple branches.

Global licenses may be purchased by governmental entities only after the scope of the license (department, agency, ministry, etc.) has been defined with, and approved by Orsenna prior to such purchase.