You can create a new page object thats twice as long as the first(assuming both pages are of equal height) and put the pages one after other in the new page.
from PyPDF2 import PdfFileReader, PdfFileWriter
from PyPDF2.pdf import PageObject
reader = PdfFileReader(open("file.pdf",'rb'))
page_1 = reader.getPage(0)
page_2 = reader.getPage(1)
#Creating a new file double the size of the original
translated_page = PageObject.createBlankPage(None, page_1.mediaBox.getWidth(), page_1.mediaBox.getHeight()*2)
#Adding the pages to the new empty page
translated_page.mergeScaledTranslatedPage(page_1, 1, 0, page_1.mediaBox.getHeight())
translated_page.mergePage(page_2)
writer = PdfFileWriter()
writer.addPage(translated_page)
with open('out.pdf', 'wb') as f:
writer.write(f)
If they are of different heights, just do
translated_page = PageObject.createBlankPage(None, page_1.mediaBox.getWidth(), page_1.mediaBox.getHeight()+ page_2.mediaBox.getHeight())