you could define an additional requiredProperty, which default value corresponds to the combination of the description and category tags values (i.e. accessible by means of the usual ${...} notation).
Therefore, considering the above cited example, you could insert the following snippet in the archetype-metadata.xml file
<requiredProperties>
<requiredProperty key="category"/>
<requiredProperty key="description"/>
<requiredProperty key="categoryDescriptionComposition">
<defaultValue>${category}${description}</defaultValue>
</requiredProperty>
</requiredProperties>
and finally name your file as __categoryDescriptionComposition__.xml.