It looked so simple, people was just doing this:

Option "TwinView" "true"
Option "TwinViewOrientation" "Clone"
Option "TVOutFormat" "SVIDEO"
Option "TVStandard" "PAL-B"
Option "SecondMonitorHorizSync" "30-50"
Option "SecondMonitorVertRefresh" "60"
Option "MetaModes" "1024x768,1024x768;800x600,800x600;640x480,640x480; 512x384,512x384"

But it never worked for me. Then I stumbled across this:

You can use a DIGITAL lcd monitor (DVI connector) and a lcd TV on S-VIDEO.

You cannot use an analog (D-SUB) monitor with S-VIDEO, is a board limitation.

(from http://vip.asus.com/forum/view.aspx?id=20070802171146593&board_id=1&model=M2NPV-VM&page=1&SLanguage=en-us)