If they are native documents (scanned direclty into Laserfiche, printed in through snapshot) then you can simply drag one over the other and it will prompt with options to combine them into a single document.
You can also see if it is a native document by the fact that it has a page count.
If it is a PDF document you can generate pages from the vertical elipsis menu, this will generate a native underlying document (the PDF can then be removed using the elipsis menu again and managing the electronic file).
If it is a document opened in some other software, it depends on the feautres of that software's merge options.