duplicate dc:identifer tags

Hi all, we are using dublin core to specify meta data associated with 
articles. The article has 2 unique identifiers - a unique document ID (eg 
346346) and a unique URL, both are required.

Can someone suggest a standard way to represent 2 dc:identifiers such as 
this. ie if I have:

<dc:identifier>346346</dc:identifier>
<dc:identifier>http://www.siteaddress.com</dc:identifier>

is there an accepted way to add attributes to these element that will allow 
me to distinguish betweeen the two.

Thanks

Tom

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