2 Gpu 6 Monitoring

Hi. I have 2 Nvidia a6000 graphics cards and I connect them to 6 monitors with display cables. but since it has only 4 outputs, I can display 4 monitors. I can’t get an image from 2 dislays from the other card. I tried to edit from Nvidia control panel, but it seems Disable, my operating system is Linux 18.04 can anyone help :)