identical key generation on multiple nodes?

Hello,

I'm trying to generate identical keys on 2 different sets of nodes

Basically I need to be able to create a new identity tag on each page
where the tag is not present, but the restriction is that the ids on
each corresponding identity tag needs to match. Page number 3 id 3
needs to have the same "identity" tag (with the same id) as page number
8 id 2 same for page with page number 4 id 4 and page number 9 id 3,
page number 6 id 6 with page number 11 id 2, etc. How do I accomplish
this in XSLT? The result tree needs to be identical to this but with
the "identity" tags inserted everywhere and with matching id's on
corresponding pages. (page.figure1 spanning 3 pages and page.figure2
spanning 2 pages). the pages that don't have a preceding page
containing and identity tag are irrelevant and don't need to have such
a tag.

<document>
  <page number="1" id="1">
      <text/>
      <text/>
  </page>

  <page number="2" id="2">
      <identity id="page.figure1" />
      <text/>
      <text/>
  </page>

  <page number="3" id="3">
      <text/>
      <text/>
  </page>

  <page number="4" id="4">
      <text/>
      <text/>
  </page>

  <page number="5" id="5">
      <identity id="page.figure2" />
      <text/>
      <text/>
  </page>

  <page number="6" id="6">
      <text/>
      <text/>
  </page>

  <page number="7" id="1">
      <identity id="page.figure1" />
      <text/>
      <text/>
  </page>

  <page number="8" id="2">
      <text/>
      <text/>
  </page>

  <page number="9" id="3">
      <text/>
      <text/>
  </page>

  <page number="10" id="1">
      <identity id="page.figure2" />
      <text/>
      <text/>
  </page>

  <page number="11" id="2">
      <text/>
      <text/>
  </page>
</document>

Any help would be greatly appreciated.

Regards
Jean-Francois Michaud