You could create a field that concatenates these two with a calculation automatically and make it validate to be unique. Or you could write a validation that checks to see if it is unique before committing.
supplier id & " | " & invoice <=== auto enter and replace
I am guess you are creating your Invoices from Order. If you do so, mark your Order with the Invoice No. Then you can not create a new Invoice if it already exist a Invoice No on Order
Wow quick replies!
Yes Taylor, my mind was drawing a blank - this sounds like a perfect quick solution, ill give it a try now.