Hi,
In the menu "System > Taxes" you will see in the listing that you have a column named "cumulative" tax.
In that same page you have button to edit tax categories and to edit tax rates.
But you can create there some new tax rules and set the new rule as a cumulative tax.
So you will have to create a special new tax category where all your product with the cumulative tax will be assigned.
It will allow HikaShop to add two taxes for these products.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.