MR. & MRS. 55 on Lata and Asha's highest notes

Bollywood's Beloved Sopranos: Lata and Asha's Highest Notes "Before the arrival of the Mangeshkars onto the filmi musical scene, female singing in Hindi films was dominated by artists with heavy, nasal voices, such as Suraiyya and Shamshad Begum. Once music directors had the opportunity to work with the Mangeshkars, things changed forever: the nasalized heavy female voices were out and the delicate soprano voices were here to stay. After Lata and Asha became established as playback singers, I would argue that  music directors of the time pushed the boundaries of their compositions in terms of range to test and showcase the virtuosity of these two exceptional talents."
They also have a theory about the popularity of the male versions when both males and females sang the same song "In almost all cases that I can think of, music directors made the female singer of a tandem song sing her versions in the same key as the male version. Because men tend to be more comfortable in the higher register of their voices than women, this practice often put the female playback singer at a disadvantage when it came to hitting the highest notes of the composition. "
See also their discussion of one of the songs in a different post where they explain the lyrics etc 
http://mrandmrs55.com/2013/05/25/panchhi-banoon-udti-phiroon-lyrics-and-translation-lets-learn-urdu-hindi/
P.S. A discussion on twin sons in 'Songs of Yore'
http://www.songsofyore.com/twin-songs/