I have a WORKAROUND which does use a script, but it's not a turn-key solution for every situation, only this instance. I tried this using a 100pp a5 landscape PDF into an A4 indesign file. Here's what I did:
1) Make a new a4 portrait size indesign file with TWO pages in it.
2) run the multipageimporter2.5 script by scott zanelli. in the first instance, make sure the start placing on doc page is on page TWO and that the positioning is from the TOP LEFT with offsets of 0 & 0. let the script run.
3) run the multipageimporter2.5 script AGAIN, BUT make sure that in the import PDF pages panel, rather than being from 1-100, it is on 2-100. Again, make sure that the start placing on doc page is on page TWO, and CHANGE the positioning – still TOP LEFT, but with offsets of 0 & 148. let the script run again.
4) ok, so now there should be a 101 page file with the first page blank, and every even page thereafter having the correct results, but all the even pages having "duplicates". these have to be removed with a script. This script is one that I've adapted from regular contributor to this forum, Jongware.
var curpage = 0;
while (curpage < app.activeDocument.pages.length)
curpage += 1;
(obviously, copy this text above into a new text file and give it a name with a .jsx ending and put it in the scripts panel in indesign. I've called this one "delete_every_other_page.jsx")
5) after this script is run, the results should be as expected.
now, as for the space of the gap wanted between the two entries... that's determined by the offset when the multipageimporter script is run on the second occasion.
hope this helps
I'd do it in 2 steps. First, use "multi page importer" or your favorite PDF importing script to put one PDF page per InDesign page. Then run an InDesign-based imposition plug-in like IDImposer to produce a "2-up" imposition. It sounds like you would want to use the Consecutive/N-Up sequencing option of IDImposer. You can specify the gap in IDImposer. Let me know if you have any difficulties.
Stephen - author of IDImposer