When using the MVC convention in Sencha Touch 2, I would recommend the following when trying to call a method called 'SomeMethodInB' in 'ControllerB' from inside 'ControllerA':
MyAppName.app.getController('ControllerB').
'MyAppName' is the name of the app you defined in the core app definition - typically in your app.js file.
According to the Sencha forums, the below is depreciated:
this.getApplication().getController('ControllerB').SomeMethodInB();
http://www.sencha.com/forum/showthread.php?158996
In fact the only way I can call the "this.getApplication()" method to even work is when calling it from my app definition file (app.js).