osihara,
The procedure to print 2 A5 pages per A4 page would be as described below.
This assumes that your Numbers document A5 pages are Portrait orientation and that your A4 pages would be Landscape orientation.
1. From Numbers, Print to PDF using the Open PDF in Preview option.
2. In Preview, Print dialog, set the Paper Size to A4, Landscape Orientation, Scale = 200% and in the Layout options, select 2 pages per sheet.
3. Print to PDF or to your printer. Preview gives you a better chance to look over the result before committing to paper. You may find that you need to change the print order to get your desired pairing of Numbers pages.
Regards,
Jerry