So you have 2 A4 pages that you want to reduce and print as 2 A5 pages on a single A4 sheet.
The dialog boxes vary depending on the printer you have but with my HP I select the following to do this.
File > Print > Properties > Features > Multiple Pages Per Sheet > 2 Pages Per Sheet.
Leaving the orientation set as Portrait since this controls the orientation of the pages printed on the sheet not the whole sheet itself.