When I read a statement in a paper that is itself derived from another paper, how should I reference it when I use it in my own article? Should I reference the original article or the second article, or both?
You should read the original article and afterwards cite it. Don't omit the "read"-step since the secondary source might have introduced an error.
BTW: You should read all the related posts, e.g. How to reference studies mentioned in a paper without having to read the original papers?