Where will you keep the southern capital? Near Hyderabad, Vijayawada, Chennai, Bengaluru or Kochi? Depending on where you keep, the other states are bound to be unhappy due to the strong rivalry.
If there is going to be a southern capital, will there be one for west, east and northeast too? That is like 5 capitals. Again, wherever you keep the regional capital, the neighboring states are bound to be unhappy too.
India has a quasi-federal system that provides enough autonomy to the states and that should be sufficient. Southern states enjoy that autonomy and identity.
While I’m a supporter of moving the capital to the coast and near India’s geographical center, I believe there has to be one capital for one India. There could be a rotational capital for the regions though, but at any point only one capital. Two capitals is like Alia Bhatt’s two states – just confusion.