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It seems to me like one framework will for serve as the core - bootstrapping, MVC stack, routing - while the other will be used to pull in specific functionality - like classes for forms or filtering or interacting with web services, etc.
As much as I prefer Zend Framework for the core - and most everything else - its use-at-will architecture argues for making the other framework the core and pulling in ZF components where you need them.
As @Adrian notes, it's easy to pull in ZF components into a project built primarily on another framework, since ZF is designed with that flexibility in mind. Other frameworks are less accommodating in this regard.
Of course it is possible and I don't see something wrong with it. For example I used CakePHP and Zend on a relatively large Project which was kind of a cms. CakePHP for the core (Routing, MVC) and Zend for some "Modules". Mainly for PDF, E-Mail (smtp), FTP and a few others that CakePHP was not that good at. With Cake it was fairly easy to use ZF components and didn't feel hacky at all.
In fact, ZF was written to be as independent as possible, so that you can use any component at any time anywhere. Yet, it would not work the other way round (Using ZF as core, and Cake components). Surely it is possible, but a real pain with no benefits. So ZF could be used propably inside of pretty much any other framework and nothing would be wrong with it, imho.