asus gaming motherboard

hi i have asus B360f gaming strix motherboard (cpu :Intel Core i3 - 7100 and 8GB of ram)and i install 2 video cards (2*Sapphire NITRO+ Radeon RX 580 4GB) on it (On PCI-Express16 Slots), the problem is that i can only get output video from motherboard (2 outputs DVI and HDMI) and from one video...