I have tried it myself in the past also and didnt get it to work either and I havent heard of a single story of a person who did get it to work.
I believe it has something to do with the onboard VGA and the first Graphics card using the same ID and therefore the computer cant send signals to both of them.
It's like calling both of your dogs 'Barky' and expecting the right one to come to you evertime you call out his name.