Optix Prime Acceleration may stop One GPU when iray render?

I finally found, why my twin 980 ti not work as double.

When I check "OptiX Prime Acceratio"n, it seems only Use One Video card when render scene.

After I check off it,, then render,,  double video card suddenly  start working and loading  in Nvidia experience monitor. 

maybe this option is only useful,, eg you have two video card, and one is 6GB another is 4GB

then your scene is 5GB,, then iray use only one GPU, with Optitex,

but if you have two or three video card, which have almost same video RAM,  

most of case, your scene can render by Both GPU, or not at all,then CPU only  I think.

(though Actually I have experienced some scene is nearly 6GB, then one  Vido card seems not work, (used for monitor too)

then after I hide some polgions of the scene,, both work again,,)