On his blog Jan Lehnardt has posted an alternative method for using the Code Igniter framework together with the Zend Framework with only the parts you need.

I want to use Code Igniter for some applications and I want to use some bits of the Zend Framework inside my Code Igniter classes. There is a tutorial explaining how to set it up. But there is a problem.

He reverses the method the other tutorial uses and chooses to go with Code Igniter for the application logic side of things, making use of the helper and autoload functionality of CodeIgniter to pull things together. He includes code to make his method clear.