So, looking at "The other one" from a computer perspective:

You'd need some kind of bootloader to run different OSs (if we consider Elliot and Mr Robot as different OS running on the same body). So there has to be a subsystem that decides when Mr Robot breaks out and when Elliot takes over.

What if it's Elliots first loss of a friend that fractured his personality and this one is the "bootloader" and Elliots key to take over.

Would be kind of Naruto-esque tbh, but it definitly would make Elliot way stronger if he could tap into "project management" Mr Robot or "Engineer" Elliot at will.

Quote me if I'm right, leave me with my head canons if I'm wrong.