Since I know someone with Mulitple Personality Disorder/Dissociative Identity Disorder (DID is the correct medical term, but MPD tends to be preferred among people who actually have it), I kinda tend to be wary of split-personalitied characters, because there are a lot of stereotypes. There's almost always an "evil personality" and they're so often portrayed as out-of-control. There are very few positive portrayals of multiple characters as people who can function in life, which sucks if that's you or someone you know!
I had played with the idea of multiple characters when I first got interested in all this stuff, but after I met my friend who actually deals with this in real life, it all seemed a little too real, and I dropped that story and those ideas for a long time.
More recently I've started developing RGB, my multiple sharpshooter. Red, Green, and Blue all have a good working relationship, but since they all have slightly different skill-sets, don't share memories, and aren't fully in control of who's "out front" when, they have to leave a lot of notes for each other and run into some obstacles making sure they're all on the same page. I'M VERY FOND OF HIM but his story is very new so I'm not sure when I'll get to do more with it. xD
If exploring alternate interpretations of "split personalities" is something you're interested in, I super recommend looking up stuff about healthy multiples! (I started when I found out about the In Essence We Agree contract and just explored from there). Even if you end up not buying into the idea of functional multiplicity, there are a lot of people in that community kind of sharing how life works for them, and different people/systems handle it differently, so it'd give you a lot of insight into how people in real life experience this sort of thing!