Mixed text should be side to side instead of on top as it would greatly decrease empty space on the pages. You might consider doing it like this: put the aya numbers for the translation directly before the text, while the aya numbers for the Arabic are directly after the text (not in a new line). This saves one line. After that split the screen vertically for each aya, but not 50/50 because translations are usually longer than the Arabic, therefore you can see statistically the ratio between them on the entire Qur'an (this can be done by counting lines in "block" reading mode) and make the screen ratio the same. So if the translation text is for example 50% longer, the screen should be split on a 3:2 ratio. This would on global scale result in much less empty space.