Steve is absolutely right. InfoPath could be useful for your PO's and your GRN's. You could then create webpart connections between the two libraries.
Assuming you are not using InfoPath and just straight forward document libraries, you could connect the doc libs using web part connections. You would need to involve a third list, which is where the dataview webpart comes in.
The dvwp displays the GRN & PO# column from one of the libraries, but provides connections to both of your document libraries. Therefore, if you click a line which states PO1, and GRN1, both libraries will be filtered.
Hope this makes sense.
Brett
Brett Lonsdale
Partner/Trainer
Combined Knowledge
MCT, MCSD.NET