This is one thing I have not yet fully understood as what is the best practice to seperate two interfaces (public and admin) of the same web app. I have couple of ideas:
Have 2 modules - public and admin:
In this case I find that sharing models between 2 interfaces is combersome.

My other idea is to treat each type of the content page as a module (articles, forum, gallery) and have AdminController and PublicController files
This approach allows to package modules and related files more closely together what I think is a plus. I am personally in favour of the second but would like to hear what other people think or what solutions have you used.

My 3rd idea is splitting both interfaces completely apart and use sort of common classes...

You can set include_path with directory to the public models as default and create abstract controller for all admin controllers with additional path to admin models if needed.
I use it once and it works pretty fine for a small project. You can also put into this controller all stuff used only by admin controllers in order to remove some code from bootstrap.

This doesn't solve my dilemma. Especially if my project is neither smalll nor exactly simple. It also introduces an extra level of indirection.

I am looking for a clean solution to seperate two interfaces and yet couple them together in a logical way. I don't personally like the idea of hiding bootstrap logic into derived controller base classes - it will confuse someone else working with my code.

What if you stored these classes instead where your Zend Library directory is and then included it in your include path?

Or if your two interfaces are going to be so closely intertwined, dont even make a second module, just make a second Zend_Layout file.