Are your stores setup in the multistore configuration or are they two totally different stores each with their own database?
Basara is correct - in that if they are two totally different complete stores NOT IN multistore configuration then you do need a license for each.
However, as I was informed previously, if you are in a multistore setup, you only have one license for your Abantecart and you only need one license for your extension. In multistore everything is in the same database.
Lee