Mirroring the CollagePages script from here for future use: https://forums.adobe.com/thread/286654#

CollatePages.js

// Complements: Planet PDF (http://www.planetpdf.com/)
// Modified by Jeff Baitis for Acrobat 9 and Acrobat X compatibility
// Improved Collate function with status bar.
 
// Add a menu item to reverse all pages in the active document
app.addMenuItem({  cName: "Reverse", cParent: "Edit", cExec: "trustedReversePages();",  cEnable: "event.rc = (event.target != null);", nPos: 0 });
// Add a menu item to collate with another document on the filesystem
app.addMenuItem({  cName: "Collate", cParent: "Edit", cExec: "trustedCollatePages();",  cEnable: "event.rc = (event.target != null);", nPos: 0 });
 
trustedReversePages = app.trustedFunction(function()
{
  app.beginPriv(); // Explicitly raise privileges
  var t = app.thermometer;
  t.duration = this.numPages;
  t.begin();
  for (i = this.numPages - 1; i >= 0; i--)
  {
    t.value = (i-this.numPages)*-1;
    this.movePage(i);
    t.text = 'Moving page ' + (i + 1);
  }
  t.end();
  app.endPriv();
})
 
// Collating pages
/*
  Title: Collate Document
  Purpose: User is prompted to select document to insert/collate.
  Author: Sean Stewart, ARTS PDF, www.artspdf.com
*/
 
trustedCollatePages = app.trustedFunction(function()
{
  app.beginPriv(); // Explicitly raise privileges
  // create an array to use as the rect parameter in the browse for field
 
  var arRect = new Array();
  arRect[0] = 0;
  arRect[1] = 0;
  arRect[2] = 0;
  arRect[3] = 0;
 
  // create a non-visible form field to use as a browse for field
 
  var f = this.addField("txtFilename", "text", this.numPages - 1, arRect);
 
  f.delay = true;
  f.fileSelect = true;
  f.delay = false;
 
  // user prompted to select file to collate the open document with
 
  app.alert("Select the PDF file to merge with")
 
  // open the browse for dialog
 
  f.browseForFileToSubmit();
  var evenDocPath = f.value;
 
  var q = this.numPages;
 
  var t = app.thermometer;
  t.duration = q;
  t.begin();
 
  // insert pages from selected document into open document
 
  for (var i = 0; i < q; i++) {
      var j = i*2;
      this.insertPages(j, evenDocPath, i);
      t.value = i;
      t.text = 'Inserting page ' + (i+1);
  }
  t.end();
 
  // remove unused field
 
  this.removeField("txtFilename");
  app.endPriv();
})