Why don't you just have two separate namespaces?  The whole reason for introducing namespaces in the first place was so that you could distinguish between similar elements from two different sources.  Trying to cram two different sources into the same namespace defeats that purpose.

WITH XMLNAMESPACES (
'urn:oasis:names:specification:ubl:schema:xsd:CreditNote-2' AS [nl-credit],
'urn:oasis:names:specification:ubl:schema:xsd:Invoice-2' AS [nl-invoice],

Drew