The upgrade of out test machine to 10.9 broke the multihead functionality.
It seems that they don't accept a window to appear on both montior at the same time, as when i drag and places each half of an window on each monitor, one half is discarded and not drawn.
This behavior breaks vncview window which is placed to cover both monitors.
This is apparently a big new "feature" in 10.9 where every screen is more or less independent. There are some comments here, including workaround:
Workaround tested and working, see http://www.cendio.com/bugzilla/show_bug.cgi?id=2682#c14
Here is the description of what has happend on the OSX side:
We'll document the issue for now (bug 4930). That documentation can be removed when/if we find a more proper fix.
Apparently RealVNC has/had the same limitation:
I found the setting NSWindowCollectionBehaviorCanJoinAllSpaces, but unfortunately, it doesn't solve this issue. It makes a window appear on all different spaces for the same monitor, but still doesn't allow it to cross monitor boundaries.