Word cannot print two A5 pages to A4 paper; it can go the other way, though, printing multiple A4 pages per piece of A4 paper. Your best option might be to change the page orientation to portrait and change the page size to A4. These two changes together will allow you to have the content from two A5 pages per A4 page. Some margin adjustment might also be needed.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]