FGO Both versions in 1 phone

What do you mean by both version?
If it's NA and/or JP server, yes you can, Because i do :)

yes, you can as long you have enough phone space

I believe NA can be downloaded no matter where you are in the world (don't quote me on that) but JP requires you have a JP device/are located in JP unless you use a 3rd party app store or get the apk online. I use QooApp which I believe is fairly commonly used to download fgo JP as it's easy to update the game when a patch goes live.