2 AMD rx 570 cards and 1 Dell Desktop

[gyro1]

Please help.   

I have 2 AMD RX570 cards and 1 Dell Desktop.  When I plug only one graphics card in the system will boot fine and I can mine ether.  When I plug the second card in, the entire system does not boot or display any video on either card.

I read something about a dummy VGA plug but these cards don't have VGA plugs. I tried plugging 2 HDMI plugs (one in each card) and rebooting and this did not work either.

How to people do this?