pair interactions in XML schema

Hi all,

For the description of pair interactions I'd like to be able to make an XML 
file like this (items renamed for convenience):

<person name='Jack'>...</person>
<person name='Jane'>...</person>
<person name='Pete'>...</person>

<acquaintance years='9'><person name='Jack'><person 
name='Pete'></<acquaintance>
<acquaintance years='1'><person name='Pete'><person 
name='Jane'></<acquaintance>
<acquaintance years='3'><person name='Jack'><person 
name='Jane'></<acquaintance>

Of course, each acquaintance between two persons can only be defined once. And 
the acquaintance of one person with the same person cannot be defined either.

Is it possible to enforce these rules with Schema?
It seems I cannot use complexTypes as keys, which would solve the problem.

Best regards, Jos