I found this article while looking through the Programmer Reference for Zend Framework 2 on running ZF1 and ZF2 in the same project.

We're very slowly porting a large application from an old system (no MVC framework) into ZF 1. The process is similar but we actually have ZF encapsulating the legacy files so we can use the ZF1 classes in the old code as we convert it. I think after that transition is done we'll start worrying about ZF2. :-)

http://framework.zend.com/manual/2.1/en/migration/zf1_zf2_parallel.html