The phrase, and what it means now
"Dual audio" is a phrase from the days when a film came as a file with two tracks inside, Hindi and English, and you picked one in the player. Today the same thing happens on the service: press Watch now, open the service, and the audio menu lists every track the title has. A dual-audio film is simply the film with that menu open, and a large South Indian release is a five-audio film by the same logic.
Switching mid-film
Every service's player lets you change the track without stopping. Open the settings or the speech-bubble icon, choose the language, and the picture continues; subtitles follow the track you chose unless you turn them off. The choice is remembered for the next title on that service.
Which titles carry the most tracks
The South Indian blockbusters on the Telugu and Tamil shelves carry the most: Hindi, Tamil, Telugu, Malayalam and Kannada from day one on the large releases. Hollywood franchises on the Hollywood shelf carry English and Hindi, often Tamil and Telugu. Bollywood films carry Hindi and, on the global services, English subtitles. The title page here tells you the original language; the service tells you the rest.
Downloads with two tracks
When you download in the service's app, choose the audio language first where the app asks; the offline copy plays with that track and its subtitles. The downloads page has the details.
