Tobiizu fake relationship au in which they never actually agreed to start a fake relationship,
Izuna approached Tobirama and offered to let bygones be bygones aiming to get him to lower his guard and dispose of him/humiliate him/steal Senju secrets (or whatever he's bored) and Tobirama Knows it.
Tobirama: Izuna's goal every time we interact is to kill me. This is no different. But I can't reject him without jeopardizing our relationship with the Uchiha.
So they become "friends" and, after the second get together that Tobirama insisted took place on a VERY public location, Izuna realizes Tobirama is onto him. But he won't come clean, because that'll mean he'd lose, and he'd very much rather chew on his own eyeballs than concede a victory to Tobirama, so he goes full on Fake Bitch and tries to trick him into actually liking him.
Tobirama tries to avoid him afterwards because suddenly Izuna became more insufferable than usual but Hashirama is like noooo, you were making friends! Don't ghost your friend! Tobirama he might start thinking you hate him!
Tobirama does hate him, anija.
Madara thinks Izuna is in love with Tobirama because he suddenly got VERY intense about him, more than usual, and he's like no you can do so much better please. He goes to Hashirama and Hashi is fucking thrilled because they could unite their families, a marriage to settle our alliance. Let me ask Tobirama what he thinks about it.
And Tobirama thinks is a great fucking idea actually. There's no way Izuna will keep this up if there's marriage on the horizon.
He's wrong. Izuna DOES keep it up, and after he sees Tobirama's little smug smile thinking he played him, he gets so angry he starts laughing like a maniac. Sharingan activated and all. Once his deranged laugher dies down he smiles "oh I'm so happy, I'm the happiest man alive!"
Now they're engaged and both fucking panicking.
The thing is, Tobirama is a controlling little freak, so even if he DOESN'T want to do this, he takes control over wedding planning and becomes insufferable in turn, tracking Izuna down to berate him because he needs to do his part as well! This is a very sensitive political affair and it cannot go wrong and Izuna I'm a sensor I know you're inside that well, come out you're gonna dirty the water.
Izuna starts to believe he was successful in his plan and now Tobirama thinks Izuna is in love with him for real and that's the worst thing ever.
Tobirama starts to believe Izuna actually meant the initial friendship overtures but after Tobirama's constant avoidance he accepted the wedding to punish him and this might be Tobirama's fault actually.
They tell nobody about what's going on.
On the wedding day Izuna breaks and hisses "I poisoned the wine!" Which is a lie, and Tobirama knows it, and he slumps in relief because that means Izuna does not want to do this. Alas, Tobirama planned this wedding for weeks with little to no sleep and invited a lot of very important people. He's NOT letting Izuna ruin all his hard work, so he drinks anyway and says "no you didn't" Izuna's eye twitch and drinks as well and now they're married.
Tobirama invents divorce a week later but they still keep on being roommates because it'd be humiliating if the other got the house in the divorce. They keep playing the friend chicken game for years to come, and build a life around the other. Izuna because eventually he starts to like Tobirama and decided to be merciful and never tell him about how this started so he could... He doesn't even remember what, kill him? Expose his fake ass? Unimportant (he still thinks Tobirama thinks Izuna meant to become friends at the beginning). Tobirama is like, I'm doing the world a favor by keeping him contained and also after so long Izuna's presence doesn't feel intrusive anymore and it's somewhat enjoyable (he likes him as well but he's never had a friend before)
Since Tobirama has no clue how normal friendships work, he follows Izuna's lead. Thing is, Izuna's naturally inclined to match anybody's freak so they actually end up following Tobirama's lead on it. And it gets. Weird.
Tobirama: hey if in tomorrow's mission you come across some enemies can you bring me a couple alive. I have a new idea I want to try
Izuna: no problem. Any specifics?
Tobirama: an earth affinity would be optimal. But if not, anything is fine.
Izuna: you got it.
Hashirama, Mito & Madara, who were having dinner with them:...
Izuna: hey when I die bring me back so I can kill whoever killed me.
Tobirama: if
Izuna: what
Tobirama: If you die. I'm about to reach a breakthrough on immortalily. You'll die when I let you.
Izuna is very touched.
Nobody even knows they're divorced.
