We tried reaching you on a private message asking for the Service Tag number to ascertain the warranty but did not receive a response. Please feel free to reply to the private message whenever you are available.

Do I understand correctly that you're swapping the cables manually between these two PCs, rather than using a KVM (Keyboard-Video-Mouse) switch?

Are you powering the monitor(s) off while switching the cables?

Is each monitor set on its own On-Screen Displays (OSD) to auto-detect the video input source?

Does the GTX 1070 card even support the 4K resolution you've set at 60 Hz, or only at 30 Hz?

Hey RoHe

Thanks for your attention.

No, I only swapped cables when I was trying to figure out where the problem was. I switch monitors between computers using the input shortcut Key from monitors.

After long troubleshooting and tests with Dell technical consultant, we figure out that my DisplayPort to HDMI cables only supports 30Hz. After the resolution,  I tried to find a DisplayPort to HDMI cable that supports 60Hz, but no lucky so far. 

@R. Battaglia Since the graphics cards in both computers has DisplayPort outputs why not use DisplayPort to DisplayPort on Computer02?

Hey Vic, the problem is that my monitors have only one DisplayPort input. One DisplayPort and two HDMI inputs. 

@R. Battaglia I don't understand how your monitors are connected. The graphics card has more than one DisplayPort output, so I would connect each monitor via a DisplayPort cable to a separate DisplayPort output on the graphics card. In your case, you only need one DisplayPort output per monitor, two DisplayPort cables, and a DisplayPort output for each monitor on the graphics card. 

Yes, exactly what I did for the first computer. But I'm sharing those monitors with an additional computer. I'm not using them simultaneously, when one is busy, I change the input on the monitors to access the second one. And the monitors don't have a second DisplayPort input, just HDMI inputs.

That's why I need a DisplayPort to HDMI cable to connect the second computer to the monitors. I would prefer to use HDMI cables, but there is an additional issue. Both video cards only have 1 HDMI output. Is it make sense?

@R. Battaglia  - Why not just use the Intel Graphics UHD HDMI port for that second monitor?

You can use both a video card port and an onboard Intel Graphics port at same time, as long as one monitor is connected to the add-in video card all the time. And no BIOS settings have to be changed for this to work. That way you won't need a DP(PC)>HDMI(monitor) adapter.

Would a multi-monitor KVM switch simplify this setup for you?

@R. Battaglia 

I understand now, sorry I missed the part about sharing the monitors. I think you need an active  adapter like this one that supports 60 Hz: https://www.amazon.com/StarTech-com-DisplayPort-HDMI-Adapter-DP2HD4K60S/dp/B01M6WK3KU?th=1

it's a lot more expensive than a passive adapter.

Thank you, guys, both ideas you brought above were great. I already have a switch but only for the keyboard, mouse and tablet. I had no idea finding a switch that includes the video input/output.

Now that I know where the issue lies, it's easier for me to buy the product that fits better!