Stefan Esser has released a patch, pulled originally from the Suhosin project's functionality, that allows you to use a bit of stealth for run the Zend Debugger and XDebug at the same time.

Those asking usually can understand why running two debuggers at the same time will lead to problems, but they don't understand why problems should arise when only one is used at a time. They want to be able to load Zend Debugger and Xdebug with the same php.ini without the need to restart the whole server just to change the debugger.

You can grab the patch from his site and install it like normal. With the patch, your PHP install will handle the rest behind the scenes.