I should have said, n-up of arbitrary page size. The default drivers in Mac OS and lots of software can print multiple full standard pages to a single standard page. Such as 2-up or 4-up with or without borders.

But by using an imposition plugin for Acrobat I could take two 5.5x8" documents and print them to a single 8.5x11 sheet, making margin and gutter adjustments at the same time for every part of the layout.

I couldn't get those two pages to print properly onto a single page with any Adobe software or driver I tried in Mac OS until I got the plugin for Acrobat.