Here's a song from 1980 Garjanai starring Rajnkanth, Madhavi and Geetha.
It's a rare duet by TKS Kalaivanan and S Janaki. Interestingly, this duet toggles between two pairs, with S. Janaki's voice being used for Madhavi and TKS Kalaivanan singing for the other male lead (
don't know who), while Rajni and Geeta don't lip-sync their lines :)
https://www.youtube.com/watch?v=wESeJrZdJ6c
Did singers like TKS Kalaivanan and Soolamangalam Murali (of Kazhugu's 'Kathalenum Kovil' fame) sing other songs for Rajni?