EDIT: I ended up installing KDE and everything worked as expected. I think that there must be something built into Unity that simply will not allow windows to stretch over two screens in the way I was trying to do it.
I'm on 14.04 Trusty with the Unity WM. The computer has two screens, each plugged into the internal display. In the Display settings panel, the two screens are stacked vertically.
All I want is for maximized windows to cover both screens. Actually, what I REALLY need is to do this from a script. I tried wmctrl, but no matter what, windows always stop at the edge of a screen.
If I drag a window across the border and then stretch it manually, it works, but this will not what I need: I need the solution to be something I can automate.
I tried this in on another computer with the same version of Ubuntu but with KDE instead of Unity and it worked. I think it would be overkill to move to KDE just for that, though.
I tried modifying values in CCSM and I tried several other settings, but nothing made windows maximize over both screens and no setting allowed vmctrl to place a window over both screens.
Surely, this must be doable. How can a script place a window over both my screens?