I think it requires an assertive Christian State, one that is strong enough locally that a Hindu rebellion can be defeated - as that is the killing blow that every Indian conqueror was scared of.
Lets try a PoD where the Roman Empire embraces St.Thomas Christians, my personal choice is a "Edict of Tolerance" - essentially, Christians are all fellow Christians, if their Patriarch is recognised by the Emperor in Constantinople. So you could have Arians in Egypt, Donatists in Africa, etc, Nestorians in Mesopotamia, and St.Thomas Christians in India.
The advantage of this is that the Egyptian and Indian Christians are likely to prioritise trading with one another, or are outright encouraged to (perhaps even subsidised by a very 'pious' Emperor.
The southern coast is a perfect place to incubate a strong Christian Tradition, perhaps with the trading Chola - a small Christian State can slowly expand and make the coasts of S.India majority Christian - which then means that they can be VERY assertive when they expand, unlike larger Empires. Another Empire can replace it if/when it collapses locally, but if they are from the region, chances are they are also Christian, which allows it to further expand - perhaps into the Deccan Plateau - rinse, repeat, perhaps you have two Empires, one on the Plateau, and one on the coasts. After that point however, you're biggest target is the Ganges Valley - the Indus would be nice, and a Christian Empire than can assert control over the coast and forcibly convert it (yeah, it probably won't be clean), can start the process there. The Ganges however is a bigger deal - there are so many people there, that you can't just easily convert them. You'd have to found new cities, and import Christians wholesale, give them the best jobs, found hundreds of monasteries along the Ganges to give them a large presence. It'd be an expensive process - and only really useful is the faith is a major pillar of the state.
But the key is a strong relationship between the premier (at the time) Christian State in the Romans, and a ascendant Indian State - probably involving trade preferences - perhaps Christian traders get lower docking fees in the Roman Empire, and are given preferential rates - but harbourmasters know to ask for a symbol of some kind from a St.Thomas bishop. It would allow those traders to be an absolute mint for any state that embraced them, as they'd be the ones making the most money from the Romans, which helps them strengthen their state.
Heck, I may be understating the ability of free missionaries after there is a solidly Christian region - if you have a solidly Christian south, it could be funding missionaries on a scale not seen in India, at any point.
So yeah, my dual PoD is a system of Patriarchates in the Roman Empire, that allow equal but different practices of the Christian faith, but equal under the law (but still preferential to pagans), and through that the acceptance and support of Extra-Roman Churches.