How to specify "coupled" attibutes

Hello All;

I have an element that takes two attributes as a coupled group, meaning
either none or all of them have to be present.
For example, if a guys wares glass, it's required that the type is
specified.
<SomeGuy wears-glass="yes" glass-type="contact-lense">
  Info about this guy
</SomeGuy>

How do I enforce this in XSD ?.  I tried attributeGroup and it's not it.

My apology if this is not the right place, in which case would someone
please give a pointer.

Thanks a lot.

Waynix.