My argument is this...had religious tensions not been created by the British Brahmin alliance after 1857 and had World War 1 and 2 not occurred, I believe a natural cultural division would have formed anyway. The Indus and Ganga have always been separate throughout its 9000 years of human history, with the exception of around 500 years with various Kingdoms and Empires, the longest being the British Empire from 1842 to 1947.
While we share similarities, I often like to compare the two as Japan and Korea.
Very similar, yet very different at the same time.
Even the Hinduism practiced in the Indus and Ganga are vastly different. Hindu communities in Pakistan worship and revere Vedic customs and gods, which was related to the Avestan religion of Persia. Whereas Indian Hindus (North Indian Hindus) revere Puranic customs and gods, which were mainly adopted from Dravidian South Indian mythology.
Vedic gods like Varuna, Indra, Mitara etc. (all of which were adopted from Avestan faith) are worshiped by Sindhi Hindus for example (Jhuelal is considered the Sindhi god, and a reincarnation of Varuna). The Kalash also revere an Indra like figure, while Kashmiri Pandits worship Kheer Bhawani, another Vedic deity.
Indian Hindus on the otherhand worship gods like Ram, Krishna, Vishnu etc. These gods are not very revered among Pakistani Hindu communities.