Expansion slot for graphic Card for HP Z220 CMT Workstation

Dear Forumer,

I'm trying to use 2 graphic card via expansion slot to provides 2 display monitors however, the system only detect 1 graphic card even I installed two graphic cards. I'm using window 10, 64 bits OS.

graphic card used

1. 128MB ATI RADEONx600 SE

2. NVIDIA Quodra 600

Please advice.

Thanks