Multiscreen has worked absolutely fine since the KDE 3 series, yo…
Multiscreen has worked absolutely fine since the KDE 3 series, you just need to make sure you set up X correct. By the sound of it you are running in what’s called "multihead" mode which has one X server per screen, this prevents windows from crossing between screens. What you want is called Xinerama (Also known as TwinView in nVidia, Big Desktop on Ati and I think merged buffer on Intel), this makes one X server run on all screens at once and allows you to move windows freely between screens.
Good luck,
KWin developer with multiple monitors