If you install eZWebIn package in a eZ5.x with eZDemo, you will get the following warnings:

Warning:
Class eZArchive in file extension/ezwebin/autoloads/ezarchive.php is already defined in:
extension/ezdemo/autoloads/ezarchive.php (var/autoload/ezp_extension.php)
This class was not added to the autoload array.

Warning:
Class eZKeywordList in file extension/ezwebin/autoloads/ezkeywordlist.php is already defined in:
extension/ezdemo/autoloads/ezkeywordlist.php (var/autoload/ezp_extension.php)
This class was not added to the autoload array.

Warning:
Class eZPageData in file extension/ezwebin/autoloads/ezpagedata.php is already defined in:
extension/ezdemo/autoloads/ezpagedata.php (var/autoload/ezp_extension.php)
This class was not added to the autoload array.

Warning:
Class eZTagCloud in file extension/ezwebin/autoloads/eztagcloud.php is already defined in:
extension/ezdemo/autoloads/eztagcloud.php (var/autoload/ezp_extension.php)
This class was not added to the autoload array.

Facing that, it makes sense to make it clearer in the documentation that eZDemo and eZWebIn won't work simultaneously, at least without warnings when generating the autoloads.

It also should be pointed in the upgrade pages that this must be considered when upgrading.