Hierarchical attributes in XML DTD

I'm wondering how to define hierarchical attributes in XML. In fact, I mean how to define attributes in such a way that wherever a specific attributes used, another one(s) could be used. For example, att1 could not be used before att2 and in every case that att1 is used, att2 is allowed.

<elem att1="x" att2="y"/>:  is true

<elem att1="x" />:  is true

<elem  att2="y"/>: is not true