how to separate pages?

In a PDF document cropping is always a mask, nothing is deleted.

This script will duplicate all pages and crop them on the left and on the right.

Copy-paste this script into the JavaScript console of Acrobat Pro, select all, and hit the ENTER key (not Return):

/* Acrobat detection and version detection, requires Acrobat 7 or later version */
if (app.viewerVariation == "Full" && app.formsVersion >= 7)
{
ProcessDocument = app.trustedFunction(function()
{
    // create a new document
    app.beginPriv();
    var newDoc = app.newDoc();
    app.endPriv();
    // get the filename of our current file
    var i = 0;
    while (i < this.numPages)
    {
        newDoc.insertPages( {
            nPage: newDoc.numPages-1,
            cPath: this.path,
            nStart: i
        });
        newDoc.insertPages( {
            nPage: newDoc.numPages-1,
            cPath: this.path,
            nStart: i
        });
        // we did this twice so that we can then split each copy of the page into a left
        // and right half.
        i++;
    }
    if (newDoc.numPages > 1)
    {
        newDoc.deletePages(0);    // this gets rid of the page that was created with the newDoc call.
    }
    // at this point we have a documnent with every page from the source document
    // copied twice
    for (i=0; i<newDoc.numPages; i++)
    {
        // determine the crop box of the page
        var cropRect = newDoc.getPageBox("Crop", i);
        var halfWidth = (cropRect[2]-cropRect[0])/2;
        var cropLeft = new Array();
        cropLeft[0] = cropRect[0];
        cropLeft[1] = cropRect[1];
        cropLeft[2] = cropRect[0] + halfWidth;
        cropLeft[3] = cropRect[3];
        var cropRight = new Array();
        cropRight[0] = cropRect[2] - halfWidth;
        cropRight[1] = cropRect[1];
        cropRight[2] = cropRect[2];
        cropRight[3] = cropRect[3];
        if (i%2 == 0)
        {
      newDoc.setPageBoxes( {
          cBox: "Crop",
          nStart: i,
          rBox: cropLeft
          });
        }
        else
        {
      newDoc.setPageBoxes( {
          cBox: "Crop",
          nStart: i,
          rBox: cropRight
          });
        }
    }
}
ProcessDocument();
}