applying vat and cst depending upon the line item

Hi Experts,

i have a scenario where we have both service item(non stock) and spare part both in one sales order and we have both vat and cst in the pricing procedure.so how can we customize,so that vat is applied on sales item only(spare part) and cst is applied only on the service item.And the customization should be applicable if we have only one line item either service or spare and respective tax should be applied with out giving an error as vat or cst missing.

thanks in advance.