To get dual-monitor remote desktop over VNC working withUbuntu, I had to install x11vnc. With the default VNC server, only half of my desktop would appear with the other half blacked out. Also, the whole two screen layout would not fit within my VNC client window.Ultimately this is the command I came up with the start the VNC server:
x11vnc -xinerama -xwarppointer -scale 3/4 -noxdamage -ncache 10
The first two arguments setup dual screen mode. The next one (-scale 3/4) scales the screen to 3/4 of it's original size (the scaling is done server-side). The -noxdamage argument prevented some annoying server errors that were appearing. And the -ncache 10 option enables client-side pixel caching, which speeds up things considerably.