PO Header/Detail Currency can't be different

Why can't the currency of the PO detail be different than the currency of the PO header?  We have a USD based company that needs to place POs and cross charge another company with a different base currency.   These are mostly non-stock POs.  The explanation we have received in the past is that the currencies may not be different between the header (BP) and detail.  But in the interest of centralizing the A/P operations, the company is looking to be able to process and pay out of one company, while still charging the appropriate company for the expenses.