For mind-streams to merge the two must exist in a shared space or a world, like two cars in a garage.
However external minds that can be grasped with wrong view to be personal for another is something conceived of internally by mind which is internal and can be grasped wrong view to be personal for one to whom it is internal & self-referable.
There is no space or world outside of the world conceived & perceived by this or that mind.
In the pali discourses, that in the world through which one conceives & perceives the world is called a world. There is no world to speak of outside of a subjective frame of reference and therefore two frames of reference can not merge.
For another person's frame of reference to merge with the frame of reference which can know it with intellect, that frame of reference of another, of which one thinks about internally as 'percipience which is external', would have to exit the world wherein it is an object of perception and be a world disconnected from the world where it was a truth altogether, before imposing itself as to make perception of the world as we know it absolete.
Two observers do not perceive the same simultaneity of events. A person moving towards light will perceive visible form before one moving away from it, visible light here & there isn't seen simultaneously as those aren't even the same particles.
Check this video on Special Relativity
Therefore if two mind-streams merged that resultant mindstream would contradict itself on account of simultaneity. It would have two pasts and be disconnected from both, eg one past where things were seen to occur consequently and another past where the same events are perceived to occur in a different succession which makes them essentially different events.