I use a multi-monitor setup with nvidia twinview, 2 identical screens side by side.

Under system settings > display and monitor > screen locker I have chosen a screen locker type of "Screen saver" and selected a screen saver (in this case, Asciiquarium, but this is irrelevant).

When running the "Test" button, a single instance of the screensaver is launched spanning both screens.

However, when the desktop is locked and the screen saver is launched, a separate instance of the screen saver appears per monitor.

Reproducible: Always

Steps to Reproduce:
1. Set up a screen saver.
2. When using multiple monitors, lock the desktop
Actual Results:  
Multiple instances of the screensaver appear

Expected Results:  
A single screensaver instance spanning both monitors