Preview shows only one page per sheet.
Before clicking Preview in the print dialog, I select "2 pages per sheet". The preview shows two sheets, with both page impressions scaled and positioned as if the first page on each sheet. The second page impression should be on the second half of the first sheet. My guess, gedit's call back method is called for each impression with an appropriate graphics context for each impression. Thus, the call back should count page impressions for the sheet, and only is issue a formfeed/page-eject after the last page on the sheet, or document, is drawn.