I'm working on a library to do that right now [1]. Currently, it's alpha quality, but you can make OpenGL apps in both, SDL2 apps in iOS and Webview apps in both. Also check out nimx [2]

[1] https://github.com/iffy/wiish

[2] https://github.com/yglukhov/nimx

I've seen it done. Since Nim compiles to C the process of getting stuff running on android or IOS is more or less the same as with C.