Chabad.org — English translation + Rashi
Torah Temimah: “שנית” builds a general rule — wherever teruah is said, there is a second tekiah with it.
English: “And you shall sound a teruah a second time” — this builds a paradigm: wherever teruah is stated, there shall be a second tekiah with it.
Paradigm from “shenis” — the word “second” generalizes the framing tekiah.
Every teruah in the Torah’s signal-system is paired with its accompanying tekiah.