Impose

Half-letter pages in, letter sheets out: two-up, duplex short-edge, cut-and-stack.

Your PDF is processed entirely inside this page. Nothing is uploaded, and this page makes no network requests at all.

Then print it

  1. Print double-sided with flip on short edge.
  2. Cut the whole stack at the marks on the centre line.
  3. Put the left pile on top of the right pile.

If the backs come out upside-down, the printer driver used long-edge duplex — flip the setting.