It’s very simple -if your computer has enough memory and resources:
Open each of the PDFs in LibreOffice Draw (two separate instances). In the document you want to add, select one page in the vertical page-banner on the left, then Ctrl+A
selects all the pages; either Ctrl+C
or right-click --> Copy
will copy all the pages.
Then in the document you want to add to; scroll to the desired page where you want to insert the copied pages (e.g. the last page), select that page in the vertical page-banner, right-click --> Paste
will add all the copied pages after the selected one.
This normally works for me, take care that pace sizes of both documents are compatible or the same. Occasionally Draw takes very long time to open PDFs and sometimes it stalls; this is normally due to heavy graphics, complex internal links or just to which software the PDF was created in.
Hope this helps,