The issue here is about styles. If you want to put the two documents
together, and retain their separate formatting, then the two documents
need to use differently-named styles.
More details, and an explanation of what's going on is at:
But, if you have Word 2002 or later, you can use the Paste Options
button. When you paste into Word, choose "Keep Source Formatting" from
the Paste Options. More information is at: