To set up multiple taxes, you need to go to the farmerwife server > Setup > Financial:
Once set, you can check the applicable taxes per client by opening the Client Contact and going to the Client Details tab:
Projects will inherit the tax from the Client. If an Object is marked taxable, then the checked taxes for the client will apply to those objects when Invoicing. Each tax is applied to the subtotal, they are NOT compounded.
Be sure to modify your invoice and/ or report templates to reflect the additional taxes.