bind:toObject:withKeyPath:options: is one-way binding?

I have two windows: Window A is loaded from NIB; and Window B is created programmatically.

Both windows have a NStextView: the attributedString of the textview in Window A is bound to the the property text of a model using IB; while the attributedString of the textview in Window B is bound to text property of the model using -[NSObject bind:toObject:withKeyPath:options:] method.

[textview bind:@"attributedString"
      toObject:obj
   withKeyPath:@"text"
       options:nil];

Here is the weird thing: the textview in Window B is indeed bound to the obj.text, but the changes in the textview is never updated to obj.text. But, if I made changes in the textview of Window A, the obj.text and the textview in Window B are updated accordingly.

So I am thinking, the -[NSObject bind:toObject:withKeyPath:options:] method is only for one-way binding. I couldn't find a clear explanation in the Cocoa documentations. Does any one have experience with this problem? How do you implement two-way binding in code?