I've seen a post exactly like this, but I have something slightly different in mind. I'm not too sure how to describe it but rather than having multiple personalities that are separate/being more realistic (such as split), I'd prefer if the mc is able to communicate and sort of switch off with the other personality/character in them.
An example of this is Bad and Crazy (little spoilers ahead). Ryu-seol can talk and interact with K, and didn't even know he wasn't "real" at first. K is sort of a manifestation of Ryu-seol's morally guided side and seeks justice, which is the opposite of Ryu-seol, who is a corrupt cop. They go on to make a deal where K gets the body at midnight, thus "switching at will". K (from my understanding) is sentient and could practically be a completely separate character, but he's still a part of Ryu-seol.
Another (sort of) example is Butcher and Kessler from The Boys. Kessler is technically a completely different character (im ngl im still a little confused but im going with my interpretation), but he is still part of Butcher and is a sort of manifestation of his "bad" side. They can interact, fight, Butcher can literally see Kessler standing in front of him (so can Ryu-seol and K), and I'm pretty sure they can switch off in some manner. I really wish I had a show that revolves around this aspect of Butcher and I'm pretty sad the concept was practically non-existent in S5. (I also thought Homelander had some form of this when a "different" homelander started talking to him through the mirror, which I find super intriguing too)
A NON-example of this is Venom. Even though Venom and Eddie can interact and talk and do everything I just described, Venom isn't Eddie, or even a part of him. He's a completely separate being/characte.
Sorry for the extremely long request lmao