From Traction Software:
PDFnUP for Linux software automates the process of 2up'ing, 4up'ing or 6up'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). The output page size can also be controlled and the 2nd page position can be offset with margin controls. Optional duplex options for outer-in for booklets, so for e.g. 2up 8 pages source you get output pages: 8, 1 2, 7 6, 3 4, 5 - 4 pages output for duplex printing. 4,1 2,3 and 1,4 2,3 duplex options also. Optional -duplicate option for duplicating pages so you get the same page 1,1 left and right output.