oh man, i think it works best as a crack au, like-
it starts when they're kids. that first time at the riverbank, when their brothers become enemies. They both have a sort of thought, staring eachother down. Something like, he's pretty. Then they clash blades, albeit briefly, and they think, he's strong.
So they go home. and Izuna, like the good kid he is, asks Tajima about it. If Madara still tried to kill that ugly senju guy, it wouldn't matter how he felt about him right? None of the other stuff mattered, as long as he always tried to kill him when a battle was called?
Tajima assumes this is izuna trying to be lenient to his older brother. It's obvious how devestated madara was over the loss of his friendship, clearly Izuna is just trying to wrap his head around the situation. So he agrees without argument or specification- yes, Izuna, so long as madara uses his full abilities to try and kill hashirama in battle, it's okay that he's got a lot of feelings about the boy.
Cool! Izuna says, and walks away, about to make an astounding number of decisions based off that response.
Tobirama, meanwhile, is having no such talks with butsuma or hashirama. Butsuma is mad that he didn't manage to kill izuna, and hashirama is mad that he tattled, so tobirama is not talking to anyone. he's spending a lot of his time on the outskirts of their territory to specifically avoid any talking, actually.
Which is why izuna is able to find him, and in the midst of their battle, lay out his arguement. Something akin to "Stop trying to kill me right now, save that for the official battles". Izuna's belief that his dad is fine with him dating Tobirama as long as they try to kill eachother during battle is nonsensical to Tobirama, but so are the Uchiha as a whole. and Izuna is pretty, and strong, and not even mad at him!
so they start dating. And they don't hide it.
Especially not Izuna. he's gotten permission, after all. So he runs around telling all the uchiha about his dates, and complimenting tobirama, and nobody thinks anything of it. Izuna apparently has lots of boyfriends out-clan; that's practical, no chance of pregnancy there! Izuna likes to make a lot of sarcastic comments about his lifelong rival's attractiveness and intelligence- insults the man certainly deserves. The idea he could be serious, or that those dates he's talking about are with Tobirama, never crosses anyone's mind.
Tobirama keeps his romance talk to conversations with Hashirama. Mostly it's him saying things like "you act like trying to kill madara is so difficult, as if i don't try and kill my beloved boyfriend every battle. get it together, loser." Hashirama, understandably, does not take it seriously. He assumes it's some bizarre argument tactic, not Tobirama's genuine situation.
Tobirama does succeed in stabbing Izuna to near-death like in canon; Hashirama, luckily, is able to save him; this nearly successful murder does nothing to dampen their relationship.
It takes them a little while to adjust to their new normal, with no murder attempts and only dating happening. eventually they start to talk about marriage. Everyone assumes they mean to other people. arranged marriages, perhaps, for the sake of their clans? Their big brothers hope they know they would never force them into something like that. matrimony can wait until they have a person they love and want to spend the rest of their lives with.
their claims that they already do go ignored. People start trying to date them- the marriage talks meant they're open to that, right? Wrong. They get treated as the unintentional attempt at mistressing/cuckolding they are, with a door slammed in their face.
eventually tobirama and izuna give up on anybody accepting their relationship (They sent out invitations to their wedding, which were treated as a ridiculous prank). they get married quietly and spend the rest of their lives living together, which everyone around them mourns as tragic lifelong bachelorhood. they must've been so marred by war that they never found place for romantic love in their hearts, how sad :( their shared grave goes unquestioned. tobirama was all about good finances for the village, it was probably a matter of practicality.