I'm wondering if it would not be better to have a single tax and having a nondeductible percentage on it.
Also this percentage could be variable, for example it can be defined on the taxed product or the party.
E.g. in Belgium VAT on car, the percentage can be computed from the usage ratio of the car.
Some taxes (purchase) are not fully deductible so this means that such tax is composed by two children. One for the deductible percentage as usual and a second for the non deductible percentage. The account for non deductible part should be the (expense) account of the line. For now we can only set it on the tax object but it is not very flexible and we should support to use the line account as tax account.