The QlikView associative engine travels CustomerKey link field, but you want to ignore that and only show OfferIDs that correspond to identical ProspectID values?
You can do this using set analysis that resets CustomerID to all those values that comply with ProspectID = OfferID. But as your PS stated that you real situation is much more complex, we may be building a solution that only works for your two Customer tables, and not for your actual data model. For example, in this case you could create a key composed of both xxxxID and CustomerKey. Or because in Sales Analysis, both IDs usually concern the same Customer, you may as wel ignore the CustomerKey link and create a link field from the two IDs.