The easiest way to do that is to use InDesign and to place your two documents on the target pages. You will need to have access to InDesign, however. It will also work with Illustrator, quite the same way.
An alternative is to print the documents “fit to page” to the Adobe PDF printer, with the paper size being the target paper size. Then you can combine the two documents. This solution does not work for macOS, as Apple disallows virtual printers.
ABAMBO | Hard- and Software Engineer | Photographer