Making one of two elements mandatory dependent on another elements results

In the NCR Header there are PO Number and Job elements, I can't make either mandatory but one of these should be filled in by the user. We've found in recent audits that many NCR's had neither element filled in.  At the very least we'd like to require the PO number entered when the NCR is cause by a supplier (We have an Element called 'Origin Code' for that).  SR 3-10217599441 was written with a response: "Development states: There is no way to ensure an either or on the mandatory property."

The ability to have the 'IF' 'THEN' would ensure that required data is entered and not so reliant on the user to remember to fill in data.  This would greatly increase the accuracy of the NCR data, reducing review time, and assisting with focused quality improvements.