In TruePOS we can set up Dual Tax on same item based on percentage of base value.
Item Master Settings:
1) Enable the Fields "Dual Vat%" and "Dual Serv%" in Item master screen.
2) And when creating the item enter the Base % of cost from which the respective taxes should be calculated.
3) Map the correct Vat And Service Tax
Configure a new formula for Dual Tax purpose see below image
Supplier master settings:-
1) Enable filed " Purchase Formula" in supplier master screen
2) Map the Dual TAX formula in the "Purchase Formula" filed while creating/Editing Customer.