If you receive a PDF that has left and right page of a brochure on just a single PDF page you would want to cut it in the middle in order to prepare it for imposition.
First page of a brochure
Second page, note that it are actually two pages
Third page is a again a single page
Now you may want to impose this PDF, but how would you do so? pdfToolbox
Select "Arrange" and "Split in half"
Execute the action
First page remains the same
Second page is only the left hand part
And third page is the right hand part
Last page remains the same
Now you have got single pages from a mixture of single pages and double pages. You are ready for imposition.
Go back to "Arrange" and select "Booklet"
Set Sheet size to A3 and select landscape orientation
Now you can simply create a nicely imposed brochure. Sheet 1.