You already have the option to add multiple tax rates, maybe for other purpose. We have a federal and a provincial sales taxes. They apply independantly on the subtotal. So, for my 120$ list price, we have 5% on 120$ (6$) and 9,975% on 100$ (11.97$). The total is 137.97$.
To open a new support ticket, first select the relevant forum and then click the "New Post" button. Example: You need support for the core plugin? Click on Core Plugin support and click on the "New Post" Button.