I'm not sure if this is possible, but let's say you want to build a CMS, but you're torn between 2 frameworks because each has some features that you like. Is it possible to create the CMS with both framewoks? Does this approach have merits or pitfalls?
Speaking strictly about the Zend Framework (ZF) I would say yes. ZF components are written to be as independent from other components as possible. I would say that it would not be very difficult (It may actually be quite common) to integrate libraries of ZF into other frameworks like Cake, Symfony, or Codeigniter.
It would really depend on which things you liked about both...
Zend Frameowrk is more like a library, so you can pick and choose components from it. Symfony also has some re-usable components you can use.
Really, you can do anything... it just depends how much work you want to do, and how willing you are to modify the framework code. Note: most other PHP frameworks are not nearly as flexible as Zend Framework (next up probably being Symfony).
This sounds very messy, so I'd advise against it, unless you use it in the fashion I've described above. It's easy to pull ZF or Symfony components into other frameworks, but not the other way around.
Hope this helps.
Is it possible to create the CMS with both framewoks?
While it's possible to use components from the Zend Framework inside other Frameworks / CMSs - this may be the case with other component libraries as well, but Zend's the most modular one that I know - It's usually not a good idea to mix two full-blown Frameworks.
Many central functions like MVC structures, URL routing, the database layer, Unit testing, error management and so on are by nature unable to be served by two frameworks at once without massive friction. Also, you're likely to get problems when one of the frameworks needs to be updated.
I would recommend to pick the framework that covers most of what you need, and try and add the missing features using plug-ins or own modifications.
Your question is too abstract. For instance Zend Framework is more to be a library of specific functionalities ready-to-be-include into Your codebase then some closed development environment like for instance Symfony framework.
Thanks to it can be ZF simply included into your codebase (and Symfony too).
Symfony Framework compared to it is closed solution depending on front server environment so you cannot use it like some other framewor's slave.
The composability of most frameworks is such that that is most likely to give you all the disadvantages of both, while simultaneously eliminating most of the individual advantages of both.
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.
You can do it. Most modern frameworks (Zend, symfony, flow) is capable to use parts of other frameworks. Symfony 2 uses Zend Logger, flow3 uses sfYaml... It is possible tu interchange some components.
But, imho, you should stick to one framework as a core, and use second only to relatively isolated subsystems. Adapter and Facade design patterns can help you to mix them together - some components / clases demands some wrapping before use in non-native environment.
For further reading: http://www.symfony-project.org/book/1_2/17-Extending-Symfony#chapter_17_integrating_with_other_framework_s_components
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.