Dual Head Issue

[drew at technteach.com]

Have Ubuntu running with dual heads, I can move the mouse from one
monitor to the other, I can drag icons from one monitor to the other,
but I can't drag a window from one to the other.  When the left half of
the window goes out the left side of the right monitor it just disappears
instead of appearing in the left monitor, and when the mouse reaches the
left side of the monitor when it is dragging a window it just stops at
the edge of the monitor.

It's probably some silly configuration somewhere that needs to be tweaked,
but which one, where?

Drew