How to point the root of the resulting tree of a transform?

[CLAVIER Remi FTRD/RTA/LAN]

To all / Have a happy 2003 Year.
My XSLT question follows.
I want to make a transformation in 2 pass.
In the first pass, I construct a tree from an XML file using a part of
the XSL file.
<xsl:template select="/" />

In the second pass, I want to transform the tree builded in the first
pass using another <xsl:apply-templates select=? /> but I don't know a
to "point" the root node of the constructed tree to make this second
transform....
Can people help me ?

Thanks