Re: Selecting the value from diff i/p XML

You key function returns the elements of Root2.xml whose id matches
those of the Root1 items.

But ultimately you want to lookup the value attributes of the
corresponding Root1 elements.

If you really want to process root2.xml as primary source, you may try
this (XPath2's 3-argument key function involved):

In fact, it's the XSLT2 and not an XPath2 key function:
http://www.w3.org/TR/xslt20/#function-key