how do i merge two pdf files into one

The short answer is over in macosxautomation.com asOS X Combine PDFs Service.

Open up the first pdf in Preview

Then from Menu

select from Edit Menu

Select Insert (your choice of)

Page from Scanner

Page from File (select your PDF file)

Blank Page

You can then save as your first file or rename it

Or drag the pdf thumbnails from one pdf into another pdf in Preview.app.

Peter