Due to the new multiple monitor handling introduced with Mac OS 10.9 (Mavericks) it is not possible to span e.g. the project windows or the MixConsole across multiple screens anymore per default.

To re-enable this feature, please deselect "Displays have separate Spaces" in the System Preferences -> Mission Control.

By disabling this preference, the additional menu bars will disappear as well and only the main display will show the menu bar - just as with Mac OS X 10.8.