Do you plan to glue those A4 pages together to form an A3 page? In that case, two A4 pages will not be enough because of the overlap between the pages.
Even if you are not gluing the pages together, you would need a printer that can print borderless documents or otherwise the areas where your big A3 was split will not get printed.
Since I didn't like some properties of pdfposter (like that it's not easy to add an overlap to glue pages or that it would output extra white pages) I wrote my own GUI tool called plakativ which should make it much easier to split an input PDF files across multiple pages to form a larger poster. You can grab it here:
https://pypi.org/project/plakativ/
If you are on Linux you can install it with pip. Windows executables are also regularly built on AppVeyor CI: https://ci.appveyor.com/project/josch/plakativ/build/artifacts
The source code is hosted here: https://gitlab.mister-muffin.de/josch/plakativ