To study I need to used at least 2 pdf files at the same time in windowed mode (so I see file A to the left and file B to the right, or file A on top and file B on bottom). I cannot do that with fossify file manager.

Firefox and any forks can render pdf, so you could open one pdf in Firefox and open one in Fennec, and window those.

Or do you just want a dual-pane file manager like Ghost Commander?

MJ pdf opens in parallel, so you can open two pdfs or the same one twice and put it in split app view too

Edit:

I tried to use firefox to open the second pdf, but it's extremely slow.

ghost commander seems a good idea but it keeps crashing.

this is an android 8 fwiw

Responding late, but MuPDF works for me for this purpose.