This question has been answered by iusemodx. See the first response.
<?xml version="1.0" encoding="[[++modx_charset]]"?> <urlset xmlns="http://www.google.com/schemas/sitemap/0.84" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.google.com/schemas/sitemap/0.84 http://www.google.com/schemas/sitemap/0.84/sitemap.xsd"> [[+items]]</urlset>
<?xml-stylesheet type="text/xsl" href="[[++site_url]][[~5]]"?>
<?xml version="1.0" encoding="[[++modx_charset]]"?> <?xml-stylesheet type="text/xsl" href="[[++site_url]][[~5]]"?> <urlset xmlns="http://www.google.com/schemas/sitemap/0.84" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.google.com/schemas/sitemap/0.84 http://www.google.com/schemas/sitemap/0.84/sitemap.xsd"> [[+items]]</urlset>