If I understand your requirements (one PDF with 2 pages), I would do the following:
Hi I have a problem configuring FFC. I need following: I have 2 pages (front + back) PDF. I have a sheet of 21 positions for Duplo cutter. I need to always have full sheet. In single-side BC it is easy - I just added job pages by multiple of 21. But this trick does not work for 2 sided BC. Can someone suggest a different solution?