How to restrict users to raise a requisition for multiple stores in iProcurement

Hi Gurus,

I need to restrict the users to raise a requisition from different stores, if the user adds an item to a cart, he should be able to only add another item of the same store else we need to throw an error

How can we achieve this? if possible from POR_CUSTOM_PKG, how do we put this validation?

Kindly suggest

Thanks,

Affy