I have a machine that I access using gotomypc, the remote machine runs XP Pro SP3 and an NVidia card. It has dual monitors with the second as an extention of the first and not a clone.
Gotomypc has a security feature where you can make the remote machines monitor go blank while you are accessing the machine so no one can see what you are doing. Somewhere in the session the second monitor goes white and cannot be accessed after the session is terminagted until it is powered off and then back on.
The problem does not seem to happen when the security feature is turned off.