Multiple tax rate area in a single purchase order: Is it possible??

Hello All,

In an ongoing scenario, business user needs to be able to prepare one purchase order with lines having 2 different tax areas (let it be A with no VAT and B with VAT) They have purchases wherein the item is not subjected to VAT but there is a service charge which is subjected to VAT or vise versa.  For example, the purchase of air ticket is an exempt transaction but the service fee that the travel agency charges is subject to VAT.

We have tried changing the tax area in the PO detail lines but the tax area reverts back to header tax area.