I have a workflow that imposes max up on a 12x18 sheet. Business cards with no bleed will impose correctly as 30up. But if they're double sided it will impose them only 27up. Why would double sided impose differently than single sided?
One sided vs. two sided for sides imaged should not affect the number up. Is it possible that there is a different value for gutter, layout values?
If not contact Xerox support, this is a problem that needs to be fixed.