Currently, there is no way to create two way cleanly bindings in QML.

Use case: You have a property in C++ and this property is exposed as some UI text input in QML. Changes in C++ need to get reflected in the QML and vice-versa. Currently, one can do:
TextInput {
id: root
text: runtime.cppProperty

Binding

{ target: runtime property: "cppProperty" value: root.text }

}

The problem I have with above is that it's not 'clear' that it's two-way binding. It would clearer if we either had a 'twoWay' property in Binding element to start with. OR we create a new Alias element (which is same Binding except it's two way).