How can I split in half a double-page scanned PDF in a single pass?

There's an excellent, free and open source tool called Briss. It is very simple, user friendly and effective. It works on multiple operating systems through Java.

Load your PDF into the app. The app will group similar pages together and lay them on top of each other. Draw rectangles on top of your pages so that they cover what you want included. It will look like this:

Even if your PDF has multiple categories of layout within a single document, Briss will handle it. For example, let's say some parts are in portrait and others in landscape. Briss will group them into different categories and let you draw different rectangles on them, and then process it all in a single pass, into a single document. Briss is very good at deciding which pages should be grouped together. It typically takes me less then a minute of manual work to get Briss started. Thus, a document of hundreds or even thousands of pages can be done in a couple of minutes thanks to this brilliant program.

When it looks good, select Action, then Crop PDF.

Truly a very neat tool.

Note: I realize this answer reads like I'm a Briss developer or something, but I'm really not. I just love the tool.

After looking on some internet answers (it is a question often asked ) I discovered that this can be done easily using the Poster option from the Print menu.

Steps (for Adobe Acrobat XI):

1. Choose Print from File Menu or Ctrl+P
2. Select Printer as Adobe PDF
3. Select Poster tab.
4. Change Overlap to 0 inch
5. Adjust the Tile scale to your needs, 100%(99%) if the result printed PDF page size is same as the current PDF page size, 75% if the printed pdf page size is half the current pdf. Tinker with "Tile scale" percents if necessary to obtain your desired result. To check the the result printed PDF page size go right of "Adobe PDF" combo-box select Properties and change Adobe PDF Page Sizecombo-box if necessary.
6. You can hit Print button when the page looks split like you desired, check the dotted line in the guiding preview:

Here is a print screen for the described settings:

Sejda.com can split scanned PDF documents in half, down the middle. Works on all desktop platforms.

Here's a short how to:

If it's a booklet scan and the pages are not in their natural order anymore it can reorder them for you too.

I'm a developer on the project. Open source.