Thank you! I've had a look at Aglona Reader, and while it looks pretty cool, I wasn't too excited about doing all text preparations manually.
However, this did point me in the right direction, and after some digging, I've decided that a combination of LF Aligner, notepad++ and Calibre works for me (all freeware) to convert two books without much manual work into one duolingual epub/azw which gives me in one paragraph some french sentences followed by their english translation in light grey.*
I'm still baffled that there are seemingly no apps which just show 2 books at once though. Aglona Reader is nice, but you have to prep the texts manually which might become tedious if they're thousands of pages long. On my computer I can just open two versions of my favourite book and put them side by side on the screen and scroll manually. I've thought there'd surely be solutions to do something like this on a tablet or maybe even ereaders, but this seems not to be the case.
*in case someone is wondering what exactly worked for me:
1. export both pdf files in Acrobat Reader as txts
2. open with notepad++, convert to utf8, do some cleanup with RegEx if neccessary (page# deletion for example)
3. let LF aligner do its thing, output as .txt
4. clean up with notepad++ RegEx again: delete unwanted last part of each line (\tfr-en\r\n -> \r\n), replace tab (\t) between 1st and second column with another character (like |)
5. import txt into calibre, convert into epub or azw.
6. calibre-editor: search and replace "|" with "</p><p class="translated_text">" to get two different paragraph classes (the other one being <p class="calibre1"> for me)so you can change the CSS properties of the french and english sentences individually
7. change text styles as desired in CSS file, for example make the english translations smaller and a lighter color.
All in all it takes about 10 minutes to convert 2 pdfs/whatever into one duolingual epub or azw.