Comment 5 for bug 61479

@aglet: It does that by default, however you have to have X starting with two monitors attached, and Twinview enabled in xorg.conf. If you don't do this, then gnome never realises that you have two screens, and puts the panels across both of them. It's a bit of a pain to set up, but once it's working you can switch layout freely, and the panels are always on one monitor only.