gistfile1.txt

# pip install PyPDF2==1.26.0
#
from PyPDF2 import PdfFileReader, PdfFileWriter
from PyPDF2.pdf import PageObject

reader = PdfFileReader(open('some.pdf', 'rb'))
writer = PdfFileWriter()

p = reader.getPage(1)
h = p.mediaBox.getWidth()
w = p.mediaBox.getHeight()

for page_num in list(range(90, 110, 2):
    p = PageObject.createBlankPage(None, h, w)
    p.mergeRotatedScaledTranslatedPage(reader.getPage(page_num), -90, w/h, 0, w)
    p.mergeRotatedScaledTranslatedPage(reader.getPage(page_num+1), -90, w/h, h/2, w)
    writer.addPage(p)

with open('out.pdf', 'wb') as o:
    writer.write(o)