Insert a next-page section break before you insert the second document at the end of the first one. The second one goes after the page break.
If the problem includes Headers and Footers, then before inserting, make sure the headers and footers in the new section are not linked to previous.
Sections / Headers and Footers in Microsoft Word (Ribbon Versions)

If you need more than this, please write back with the specific formatting that is causing problems.