Link two DTD files?

Hey guys, I'm trying to linke two DTD files together for an assignment. We have to split one DTD file into two separate files, one containing all the elements, the other containing all the attributes and we're supposed to use external parameter entities to link the attributes to the elements DTD but I'm having issues with it, could anyone help me with the syntax?
Thanks

The types of general entities include:

* INTERNAL (PARSED)
* EXTERNAL (PARSED)
* EXTERNAL (UNPARSED)


INTERNAL (PARSED) GENERAL ENTITY Declaration:

Internal parsed entities generally reference text. The correct definition is that they refer to data that an XML processor has to parse.

<!ENTITY name "entity_value">


where:

* entity value: any character that is not an '&', '%' or ' " ', a parameter entity reference ('%Name;'), an entity reference ('&Name;') or a Unicodeglossary character reference.

Example:

<?xml version="1.0" standalone="yes" ?>
<!DOCTYPE author [
<!ELEMENT author (#PCDATA)>
<!ENTITY js "Jo Smith">
]>
<author>&js;</author>


Hope that helps you.