It all depends on what your goals are with a "cross platform" environment. If you're hoping for a single code base that compiles to ios and android, you will end up with a Cordova-style solution, and I don't think you want that. On the other hand, Xamarin has done an impressive job with their "Xamarin Forms" project, which produces very different UI, depending on the platform, but then you're tied to a third party solution, and you will very quickly realize that giving up that kind of low-level control comes at a steep cost (animations being a biggy there).
At the moment, there is no system in RubyMotion that allows you to quickly and easily share code, and I would argue that this is actually a good thing, because we shouldn't be running blindly into a half-baked solution. I would argue, too, that the number one goal of RubyMotion is to support native application development using Ruby. Yes it does support multiple platforms, but that doesn't mean it's automatically cross-platform
What you can do right now is share your pure-ruby code, which is usually the model layer of the application. But even this is a bit tricky; you'll need to keep that code in a separate folder (I recommend including it as a gem). If your model layer includes HTTP requests, you'll need to have a platform-specific file to perform those requests. You can also write "Routers". In the VIPER system of development, these are the classes that define how to get from one screen to another. On Android, they are very similar to an
Hope this helps!