Application Unity3D only work a GPU

Hi all…
I am developing a game with Unity. When I make a compilation I realize that my game only works with a GPU, when I have two (My graphics GeForce 690 is 2GPU’s).

The first thing I thought was that it was a configuration problem in the graphics options. And in the Nvidia Control Panel -> 3D Settings -> Control 3D settings -> Mode multi-GPU rendering, I have looked, and my application made Unity3D does not exist the option of activating the Multi-GPU mode.

I checked with other games such as The Witcher III and Multi_GPU if that option appears.

What is due this?

How can using code (Script from Unity3D) make the application use both GPU’s overload rather than merely a single GPU?

Sorry if this post is not here, but did not know where to put it

Greetings and thanks in advance.