How can plugin systems be designed so they don"t waste so many resources?

I am trying to build a basic plugin system like the kind you often find in a CMS like WordPress. You have a folder of plugins which tie into the main system's operation through event notifications using an Observer or Event design pattern.

Lightweight CMS in PHP

I am building a site which will require some very limited content management for a client. There are only a few areas of the site which will require the client to be able to update the content themselves.

A CMS on 2 DIFFERENT frameworks?

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?