This is probably not something I would want to do in Omnis, but ….
You can store a page setup for a printer in a report. That page setup stores detailed printer specific information, for example how many pages shoud be printed on one page, which printer are you printing to, what tray etc.. If you now set the repeatfactor to 2 in your Omnis report, two pages will be printed, normally on two pages. If you tell the printer driver to print two pages on one A4 page, your two pages will be printed on one page, saving you paper. It is a similar principle to printing one paper page front and back. You cannot tell Omnis to do that for you, but the printer driver knows how to do this, so you load the page setup with the correpsonding printer settings before printing your report, and let the printer driver do all the magic. Not all printer drivers support this, so it makes little sense to have Omnis define this directly.
Here some links that might help explaining how load page setup works: