To add multiple tax for the product, please go to Products & Service section >> Click on Edit (in which you are looking to multiple tax) >> Click on Add Multiple taxes >> Select the required tax % (you can also select the default tax to be applied in Invoice) >> Save >> Update Item.
Note : You can add maximum 3 tax slab for a product.
While creating Invoice, you can select the tax percentage according by clicking on tax percentage shown under the total amount.
Note : You can also create invoice by adding a single product twice selecting different tax percentage.