For the Pro version it s one license for each system, there is also a family version,
http://www.avast.com/eng/avast-professional-family-pack.html for up to 10 licenses, but that I feel might be a bit much.
I don't know if that would be a cheaper option than just buying two Pro licenses, well I do know I just did a little check using the above link and clicked purchase license for one year, see image for result which is for UK. So it might well be more economical for you to use this avast pro family version, possibly more so if you were to buy a two or three year option.
So having purchased a single Pro license, I would suggest you contact sales (at) avast dot com and they may be able to help it trying to convert that to an avast pro family version.