Two GPU's only 1 render tile

I’ve noticed that even though I have 2 GPU’s (1080Ti + 1070) it only renders with 1 tile at a time, instead of 2 tiles.

In Settings, I’ve ticked both 1080Ti and 1070. They are also both set to “Display”. I don’t know why as I only need one display (I got a support to insert the cards).

Is it a failure of the 1080Ti?

One more thing, sometimes my 1080Ti is flashing red/green/blue physically on the card, very fast and constantly. Then it stops, and it repeats the pattern. Sometimes it just changes the color of the “Geforce”-text without flashing.

Is this an indication that something is wrong?

My monitors are both plugged into the 1080Ti, and displays just fine.
Though the slow rendering with only 1 tile worries me, along with this flashing LED.

Is there any hardware changes that needs to be done?

I’d assume only single card is used by Cycles.

a.) Use diagnostic tools (ie. GPU-Z) to check card’s performance
b.) check the manual to see what LED flashing indicates
c.) use NVidia forums

PS
IIRC, no GTX card can be set to run headless - it’s in the drivers - on that note use NVidia drivers (remove those which windows automatically installs)

PPS
Could also be OS, drivers, MoBo…

Put some effort to know your stuff in detail & Next time provide more significant details (OS, blender versions, images, tests performed…) or visit/consult with your local PC service provider

But why is only single card used by cycles, when both are ticked/showing up in Blender?

It has been tested with GPU-Z, as far as I know, and it showed no problems.

OS: Win10.
Blender: 2.79

The flashing LED’s on the GPU are… strange to be sure.

First recommendation is test Blender with each GPU. See if it renders properly with just one card in the SYstem… record also the render times for each GPU.

Then again try with both GPU’s and see what render time you achieve with both of them in the system.

I did have somewhat similar problem with my AMD cards, due to drivers incorrectly numbering the cards. It seen both, i was able to select both, but only rendered on one. That was a bug that I reported to Blender team and within few weeks they fixed it.

So if the individual card tests work, but two in the system dont’ then it coudl be just blender not seeing something.