Make zombies & humans coexist
Preleminary information
We are in a post apocalyptic world, the human population is drastically reduced, and radiation "gave birth" to the undead. Long story short, in this zombie world, if there is a concentration of zombies you'll find a lot of radiation. But not always the other way around.
Zombies are also very smart and organized. Some have kept their human selves, a lot have even improved in some areas. Lots of them are just basic zombie grunts that can barely say "braaains".
The subject of interest
I'm trying to have a city that has a human side and a zombie side, pretty much like they did in World of Warcraft with the city of Shattrah.
That would be a very cool hub for quests, dialogues, and a general good source of "reasons and ideas" for me in the future. I would really like to have this.
But I need a reason for it to have happened, a way to make it work because of radiation, and a reason for both sides to not attack each other relentlessly.
Practically, the city is shaped like an egg, north side is human, south side is Z. There is a very large road that crosses horizontally which is the frontier.
I'm thinking all the radiation would be only in the south side, and there would be a massive lead/concrete wall or something similar to help reduce the radiation to the north side. That and also some distance. We'll assume that they live with very high amount of exposure but would rather have that for a while and not have to deal with the outside of the city, which is far less controlled and you would most likely be eaten.
The middle road could be some kind of "no mans land" where nobody attacks each other but everyone can interact and trade (quests, etc.). I think this is really cool, and it'll make my players very curious of the zombie side. Which I could use later.
Now that area would be dangerous because of increased radiation, but there are practical ways to handle it (hazmat suits, not staying too long, etc.) That can be dealt with just like having a city near an irridiated area.
The real question
I don't see any reasonable reason this scenario would be viable, yet, I feel like it could be done. How?
I was thinking of some kind of "forcefield" (like UV lights) so zombies wouldn't attack, and zombies would have trading supplies that human needs or something. 934
But that doesn't really work because my zombies are daylight friendly, and also this all feels... not real.
Do you think this is possible? In what context could humans & zombies coexist?
Sorry about the wall of text for such a simple question. I wanted to give as much information as possible to get the best answer possible.
Thank you very much for your time.
EDITS
Adding some more information to help provide an answer :
Everyone was human once, most of them were injected with the brand new cure for old age, which eventually had a bad chemical reaction and lead to aggressivity & death, as well as lots of radioactivity. This bit is probably scientifically ridiculous, but is already established in the game.
The city has not been discovered yet by my players, so it can be anything. Right now I like to believe that the people that died and came back alive with self awareness/intelligence just moved to another district because it was easier for everyone, the main reason being the radiation that the undead "radiate" and would kill the living.
Slowly the undead lived on their side and the living on their own side aswell.
Some zmobies lost intelligence and just run for brains. Some a stronger, some have special abilities. I'm leaving myself an infinite scope of possibilites. They don't throw fireballs, they don't fly, they don't shoot lasers through their eyes, but some can have XRay vision or incredible hearing. I don't know how I define this, but you might get the general idea. It's not "too" supernatural, but it's a bit supernatural.
Right now I'm buidling the city from scratch, and from what I've read, I need both sides to have an incentive, a need to each other, otherwise it won't hold.
They're not always uncontrollably aggressive, some are, but in the city we'll say that most aren't (or are kept under control by zombie police force?). But if they're generally stronger (which they can be considering they have powers and humans dont), they still need a reason to not just bluntly attack the human side. This is the bit that troubles me most I think, as right now, this is what makes my city be plausible or not. 934