How do texture one side of a wall during run-time?

I have a wall which I allow the user to paint on with a texture. As the wall is two-sided, the user is able to texture one side with say, a brick texture and the other side with another type of texture. This is all done during run-time.

Is there anyway I can swap the material of just one face during run time?

Easiest way would be to just make it from two seperate meshes inside the model, then it's just a case of texturing the right one - that also lets you add seperate colliders to each to make picking easy

The other way uses two UV sets, but that's a bloody pain to maintain from my experience