Equirectangular shader, how to support Stereo

I have equirectangular images and i use it with a skybox equirectangular shader (source)

Now i need to use stereo images.
In previous versione of Oculus SDK there are two camera,
but now with recent SDK of Oculus (but also in SteamVR) there is only one camera.

SteamVR_Render have "Left Mask" and "Right Mask", i use it in other cases, but i don't understand how i can use it with skyboxes.

It's possible to have two skybox under different layer for...

Equirectangular shader, how to support Stereo