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.