lcd and crt

i'm using inspiron 2560 and recently added a viewsonic 17" monitor so that i can 'spread' the contents on my 14" screen to the monitor. i used to be able to do that by holding fn and f8 but now i cannot do that .instead i am seeing these:

1. both screens are showing exactly the same thing

2. only one screen works and the other screen 'shut off'

3. after frequent holding fn+f8, the mouse disabled and i had touse the screenpad.

please help.