ASUS M4A89GTD PRO /USB3 and my VGA is not working well.

Hi Guys , I am Toxel.

I got my new videocard ( ASUS HD7770-2GD5 ) and i have a problem, that it isn't working well.
I have this Motherboard : ASUS M4A89GTD PRO /USB3 . (it has a VGA Switch card , that i have to put in the 1st VGA slot if i want 16x speed )
When i put my VGA to the 1st VGA slot( the top ) , it is running perfectly , but only 8x speed .
And i want 16x speed , but i can't do that because , if i put my VGA to the 2nd slot (the bottom ) , it doesn't do anything. There is just a black screen and i don't know why.

I hope i was understandable , my english isn't too good