In order to keep the layout the same, I would recommend
inserting a section break at the end of the one page
template document and using Insert, File to insert the
contents of your long document file.

For styles, that may be more difficult depending on how
you have the styles set up. If both documents contain the
same style names but different formats, you will need to
change the style names in one of the document before you
combine them. Styles are defined for the entire document
so you cannot use different formatting for the same style
name.

If your styles in each document have unique names, you
will not have any problems when you combine the
documents.