Chengalpattu-Villupuram section had parallel BG and MG tracks. It was not dual gauge to the best of my knowledge.
There were very few dual gauge sections in the south. Madras Beach - Royapuram (Only dual-gauge electrified track in the country, with electrified MG trains), Bangalore City - Yesvantpur - Yelhanka (dual gauge MG-NG), Ernakulam Junction (only the station yard and a short stretch around it), Secunderabad-Hyderabad (For a few decades before opening of the SC-KCG section) and Vijayawada-Krishna Canal junction (Only the Krishna river bridge).