1)
What is the meaning of "va'Tivni Lach Gav"?
1.
Rashi: You built a tall Bimus (a rock set to worship on it) or Mizbe'ach. Va'Tivni is an expression of building.
2.
Malbim: [Achaz] increase to totally abandon Hashem and publicly worship idolatry. He was like a harlot on the road, Hefker to all, until building a place to lie on it.
2)
Why does it add "va'Ta'asi Lach Ramah [b'Chol Rechov]"?
1.
Radak: The matter is repeated [in different words].
2.
Malbim: It is a high place to lie, so all will see and know that here is a place of Zenus. Initially you did so in every street, to be Mezanah with men of the city.