Strictly speaking, it's not possible to print two full-size A5 sheets on A4.
That's because there's an unprintable margin where the sheet is pinched and pulled.
The A5 pages have to be scaled down to fit (and the margin trimmed out).
There is a trick, however.
That's first printing the bottom half of the sheet (where there is no margin).
Then turning the sheet over to print the other half (let dry first).
By whatever means your program or hap can use, explained elsewhere.
Most A5 documents can be cropped down to fit by 2 on an A4 sheet in one print.
But that's ugly doing for photographs.
A4 = 21 cm x 29,7 cm = 2x2×10,5×14,85 ≃ 4×10×15 = 4 photos.
The above turn-over method can be used to print 4 10×15 photos on one A4.
At the cost of cropping or shrinking each photo by a mere 0.75 mm in height.
Devised in Wallonia.
1It's not so simple, e.g. I have a page border on the A5 that now wraps the whole A4, and alignments to page sides etc. all for a loop. I think it will be easier to fix the layout by taking an image, or PDF, and printing two of them per page. – ProfK – 2014-01-24T02:27:04.207
@ProfKaos, if the layout is a problem you could try adjusting the margins, or even better yet, convert the page into A4 and use the two text column layout. – Adam – 2014-01-24T10:26:44.077
Yes, I think converting the page to A4 is the smoothest way for now. Everything else I try, it halves the size of the A5, and prints 2 in an A5 size on the A4 paper. – ProfK – 2014-01-24T13:46:18.370