You are equating bifurcation of states for political & administrative reasons, within the same country, to
splitting a country into two !
When a country is split, crimes committed against those who are migrating to another country will not be prosecuted by the home country, and will not be possible to be prosecuted by the split country.
Land, Houses and other immovable assets will be stolen from those who are forced to leave respective countries.
Rapes, looting and arson are rampant, on both sides.
In the case of India and Pakistan, the division was on explicit religious lines. Islam has an outdated theology of being the only true religion and all other religions are false (as is Christianity). This theology forms the basis (whether that basis evolved later or is advocated by islam is a secondary issue) of the idea than non-believers can be treated inhumanely and are essentially no different from animals.
In essence, Partition of India was not "drifting apart". It was a violent tearing apart of countries and communities. The origin was from the muslim community, whose political leaders realized that for them to seize power, a democratic setup with majority Hindu population will be impossible. So, to preserve their power and egos, they split the country, aided by pacifists like Gandhi and Nehru.
To note, Pakistan has one of the most fertile lands on earth. Abundance of natural resources and a strategically important location.
Also to note, the plan was to destroy Hindu civilization as a whole (a historical dream of Islam. Search google for Gazwa-e-Hind). Jinnah proposed a 10 mile wide corridor, cutting across the heart of India to allow road access to east pakistan (now Bangladesh)
Partition of India is the manifestation of the idea of Gazwa-e-Hind. A strike at the Hindu India.