I have a stylesheet XSL that includes another one by means of the
instruction <xsl:include href="XSL_U2.xsl"/>.

The second stylesheet takes the values from a different XML file.

No, it doesn't. Do you mean the templates from the included style sheet
*should* be applied to some XML toher than the input of the main style
sheet?
You can use the document() funtion to access another XML file. You'll
have to get the URL somehow, either computed from content of the
primary input document or passed as style sheet parameter.

> Since I can make reference in the second stylesheet to elements XML of the
> first stylesheet?

I can't quite parse this sentence. Would you mind restateing your
problem?