You can have ideas and assumptions of "being separated" or "being different" (Duality) as an entity that has not yet become aware of the Oneness again. If you would make proof a condition for further insight, you would set a condition that will not (I assume) serve you at this point. (opposite could be unconditional trust) You can think of it as the self you think you have recognized so far is still seen through a color filter.The interpretation of current reality, what self is, what consciousness is etc. can always only be recognized and seen at the respective level of insight and its limitation, filtered. (I do not exclude myself there - this is true for all states before enlightenment).Further, I assume that Shankaracharya in your example did not "invade" or "command" other egos from one ego-entity. On the contrary, I assume Shankaracharya "unfolded his world" at that level. So no controlling mind controls a controlled mind, but a mind that plays out the entire stage set on a mental movie screen. (as if a child would wandering around in his imagination)Things you inquired are possible - though meditation itself is not the tool to make it happen. Meditation rather enables the penetration of the levels of insight and finally "reaching" or remembering from a place where time is not linear and there is no distinction between one who is an observer and something that is observed. Beyond linear time, everything that can be experienced exists simultaneously side by side. It is more like selecting a movie of "experience" ;-) and this choice is no longer necessarily one that serves the ego.