Error after installing 2 graphic card 4870x2

[Nevado7]

Hy a few weeks ago i asked about installing a HD4870x2 Monster in my new MP4,1 just for using in windows.
I ordered those 2 power cables to connect the gpu to the mainboard but when both cards are in the mac (gt120 and hd4870x2) the system won´t start to any OS either mac nor win7.
Does somebody have a reason!?!

 

[Cindori]

what slots are the cards in?

 

[Nevado7]

Hy Cindori,

i use both 16x Lines. The first one, is still the gt120 and the second slot with 16x lines i wanted to use for the radeon.

Should i use another constellation!?

 

[Cindori]

try set your windows disk as startup disk, then shut down, remove GT, put radon in lowest slot and boot

 

[MacVidCards]

Did you get an 8 pin adapter for the 4870x2?