PDFnUP software automates the process of 2up`ing or 4up`ing a pdf document, so for example with a/b 2up page 1 is on the left and page 2 is on the right, with split 2up (4 page document) page 1 is on the left and page 3 is on the right, then page 2 is on the next page left and page 4 is on the right.
It has an optional -down parameter for 2up`ing downwards on a page rather than across the page (left, right)