Texture Atlas and planar UVs

I've made a plane mesh procedurally in Unity, and I want to use a texture atlas to assign different textures to different faces of this plane.

To assign a simple texture I can just use code like this.

But I can't figure out how to assign a texture from a texture atlas, like, if I have a texture atlas with 2 textures, how do I assign the first texture?

The point is that I've created a plane procedurally because this way I can create a plane with the number of faces I want at any time. This is why the UV mapping has to be flexible, like the example in the link above.

Thanks in advance.