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.

Sheet 2.