From the Acer 20in, open Catalyst Control Center.
From the Graphics Drop Down, select Desktops and Displays
In the lower right, you should see both displays (Acer and Samsung)
Right Click the Samsung display and select Disable.
If prompted with the "Do you want to Keep these settings" select yes.
Right click the Samsung again and this time, select Extend.
Once your display is extended across both monitors, you can then move the media player over to the secondary display for watching movies.