I've wondered about that too, forever.
Are you really sure? It doesn't seem to be addressed anywhere, and it does look very like them, but I always took that to be a dramatic license. I thought, and still think, that they cast people who simply looked very like them, for effect. It does not look exactly like them, but records show that this was shot by the second unit, and I've heard no mention anywhere of Newman or Redford being involved.
I'm not saying 100% that you're wrong, I'm still not absolutely sure myself, but I relly don't buy that its Newman and Redford. They are very distinctive. That looks like lookalikes to me, and having the stars do it themselves seems to me both needlessly expensive and artistically wrong.
But I too would love to see a credit and know for certain. Any sources?

Yes. Look again.

Are you sure?

If you look carefully it is Redford and Newman. I wondered about the same thing, but since it was in sepia it could be misleading, but it is definitely Redford and Newman.