When left idle monitor 'displays' black

[Gron113]

Basically the title of the thread above, but I'll go in a bit more depth.

I want my computer screens to turn off when idle for 15 minutes. The one connected to my GPU turns off fine, no problems there. However, the second monitor connected to the motherboard still displays. More specifically, it doesn't display the desktop or anything, it 'displays' black. Like, black pixels. It's still on. Any way to fix this?

 

[Roland Of Gilead]

connect both monitors to your GPU, It's because it's on the motherboard connectors the second one doesn't sleep.

 

[Gron113]

Is there some way of not having to connect both to the GPU? This same setup was working fine a while ago but upon trying it again it doesn't work, the monitor connected to the motherboard doesn't power off