In OS X 10.9 Mavericks, Apple changed multiple display behavior in the most dramatic way since 1987. By default, each display now has its own menu bar. Windows can be dragged from one display to another, but they cannot span displays. And putting an application into full screen mode affects only one display. You can use other displays, either with multiple applications, or with other full-screen applications. Apple describes this behavior as “Displays have separate Spaces”, and indeed the effect is almost like having multiple Macs connected to a single keyboard and mouse. I find that I like the new behavior, as I tend to put different applications on different displays.
This is a very different way of working with multiple displays, and might cause some confusion and annoyance for your users and support staff if they aren’t aware of the change, or if it affects their workflows. If you or your users prefer the “old” pre-Mavericks behavior, it’s simple to revert. Open the Mission Control preference pane in System Preferences, and uncheck “Displays have separate Spaces”. A logout is required to effect the change.
If you think you need to manage this for your users, to revert to pre-Mavericks behavior, set “spans-displays” in the “com.apple.spaces” preferences domain to TRUE.
% defaults write com.apple.spaces spans-displays -bool TRUE
You could probably use MCX or a configuration profile to manage this as well.