Coordinating dependent PO's that are processed by outside suppliers

Currently, we have a few items that get purchased from one supplier and get drop shipped to a second supplier to get additional work done.

We are having issues ensuring that the first supplier items are getting received prior to receiving them in from the second supplier.

Items are on separate PO's in some cases and manufactured parts with subcontract operations in other cases

What happens is when the second PO arrives receiving isn't sure what to do w/ the material or has no idea that its associated to another PO thats still yet to be received.

How have you handled this type of challenge?

Perhaps I could add a new field on the PO header allowing entry of a "Child or Parent" PO #? Then in the receiving application if the child has arrived before the parent pop-up information about the parent has not been received.