You mean, you want to create an output file that contains two separate XML documents concatenated end-to-end?
You can't do that with standard XSLT, because it's a very non-standard arrangement. Create the two documents separately, and then concatenate them with some other non-XML-aware utility.
Michael Kay
http://www.saxonica.com/
Author, XSLT 2.0 and XPath 2.0 Programmer's Reference