[Foxka]

Could be either. Here's some questions;

What CPU do you have?
What GPU do you have?
What model of PSU do you have (if you can't provide an exact model, provide the wattage)?

Exact models are going to be extremely helpful here. Even moreso if you can link directly to the PCPartPicker/Amazon/Newegg/etc. page.

And some thing to try;

If your CPU has integrated graphics, disconnect your GPU and plug in the second monitor. Does it work?
Is there anything that happens before the monitors flash that would indicate this being a recurring issue? In other words, do you think you can replicate it?

As it stands, I have to agree with @RussEfarmer and say that it's likely that your card is drawing too much power (as opposed to either failing), but assuming one was failing, it's liable to be the GPU. That's not me saying it's the GPU, though! Give us an answer to the questions I asked above and we can get a better picture.

PSU - https://pcpartpicker.com/product/rCH323/...p1850bbefx
GPU - https://www.newegg.com/Product/Product.a...6814202229
CPU does NOT have integrated graphics.
Monitors have occasionally frozen, turned off, then restarted however this was also a problem with my old GPU. (R9 270)
My system also occasionally gets bouts of 100% disk usage but i think this is unrelated as its a common problem with w10