well.. I really hope it'll be possible to have floating windows even though you're in full screen mode.. (would be really strange otherwise, since it works with Sonar maximized)
but why would you want to stretch out the window to both monitors, anyway? doesn't the gap between the monitors make it hard to work in a window that stretches from one monitor to the next?
I just have the main window (track view and inspector) on the left monitor and multidock (with browser, console, pianoroll ect.) on the other one