This bug seems to be solved in unity 4.28.0 already.
The only related (minor) flaw I can detect is that if you try to dock a window on the right side of the left-hand monitor, and you move your mouse pointer a little bit onto the other monitor, the indicator shows it will be docked on the left monitor, but releasing the mouse button docks the window on the left side of the right-hand monitor. That's no biggie though, and I guess I should be filing a different bugreport for that.
This bug seems to be solved in unity 4.28.0 already.
The only related (minor) flaw I can detect is that if you try to dock a window on the right side of the left-hand monitor, and you move your mouse pointer a little bit onto the other monitor, the indicator shows it will be docked on the left monitor, but releasing the mouse button docks the window on the left side of the right-hand monitor. That's no biggie though, and I guess I should be filing a different bugreport for that.