import PyPDF2
from formlayout import fedit

def main():
	paths = [('Input', ''), ('Output', '')]

	pathsRead = fedit(paths, 
	                  title="Interleave pdf",
	                  comment="Enter the full path to the source pdf and a path to output the result."
	                  )
	# Full path to files should be specified eg C:\Users\Sam\Documents\Input.pdf and C:\Users\Sam\Documents\Input.pdf

	document = PyPDF2.PdfFileReader(pathsRead[0])
	writer = PyPDF2.PdfFileWriter()

	for page in document.pages:
		writer.addPage(page)
		writer.addBlankPage()

	outputStream = open(pathsRead[1], 'wb')
	writer.write(outputStream)
	outputStream.close()

if __name__ == "__main__":
	main()

import PyPDF2
from tkinter import *
from tkinter.filedialog import askopenfilename
from tkinter.filedialog import asksaveasfilename

class Application(Frame):
	def __init__(self):
		self.input_path = None;
		self.output_path = None;

		Frame.__init__(self)

		self.master.resizable(False, False)
		self.master.title('Interleave PDF')
		self.grid()

		self.button = Button(self, text="Select input", command=self.load_file, width=12)
		self.button.grid(row=1, column=0, sticky=W)

		self.button = Button(self, text="Select output", command=self.save_file, width=12)
		self.button.grid(row=1, column=2, sticky=W)

		self.button = Button(self, text="Interleave", command=self.interleave, width=12)
		self.button.grid(row=1, column=3, sticky=W)

	def load_file(self):
		self.input_path = askopenfilename(filetypes=(("Adobe PDF Files", "*.pdf"), ("All files", "*.*")))

	def save_file(self):
		self.output_path = asksaveasfilename(filetypes=(("Adobe PDF Files", "*.pdf"), ("All files", "*.*")))

	def interleave(self):
		if self.input_path and self.output_path:
			document = PyPDF2.PdfFileReader(self.input_path)
			writer = PyPDF2.PdfFileWriter()

			for page in document.pages:
				writer.addPage(page)
				writer.addBlankPage()

			outputStream = open(self.output_path, 'wb')
			writer.write(outputStream)
			outputStream.close()		

if __name__ == "__main__":
	Application().mainloop()

Replace formlayout GUI with tkinter
Replace formlayout GUI with tkinter

Separate buttons for selecting input and output, and for running
the interleave procedure.
