XSLT 1.0 does not support multiple output documents.
You can use an extension to do this. The children of 'c:document' become the contents of the output file. c:document and its descendants will not appear in the "primary result":
<xsl:transform version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
extension-element-prefixes="c"
xmlns:c="http://exslt.org/common"
>
<xsl:template match="slide">
<c:document href="slide-{count(preceding-sibling::slide)+1}.xhtml">
<html>...</html>
</c:document>
</xsl:template>
</xsl:transform>