Even if Pakistan and India were to jointly produce what can they develop:
* India's LCA project (4.5 gen plane) still not completed. They have difficulty producing engines. 5th gen is a long shot (aka MCA project) at this moment for India.
* What can Pakistan contribute? Pakistan has neither technical expertise nor the financial muscle! (no pun intended)
Even China with its vast economic resources still stuck at 4th gen planes with newly built aerospace testing facilities.
But then again, Pakistan has fought all its wars against India. Pakistan's aerospace industrial capacities are much smaller than India (no pun intended). So in case of a war with India, do you think India's HAL will send figher jets' parts to Pakistan! IMO, the strategically the route Pakistan is currently taking is best route. Atleast it is getting some help from China, which has supplied it parts during the wartimes.