It mainly boils down to sandboxing. If you want to program your own work around look into postMessage on the internet and this forum.
If you can adopt your project to custom behavior you could also use Hype GlobalBehavior (Custom Behavior Extension). Just include the Hype GlobalBehavior code in both hype files and then send a custom behavior like gotoSceneA and listen to it with #gotoSceneA on the receiving end.