@klemi: I have my doubts whether it is possible to dynamically switch display configuration using separate X screens. Can you use nvidia-settings to switch on the fly from your current configuration to a single screen (without restarting X)? And to TwinView? It would be interesting to know.
You may want to choose the TwinView configuration in nvidia-settings.
If you really experience a system or X crash, please be so kind to file a bug at https://bugs.launchpad.net/disper/+filebug