You know the gist, Desmond dies and travels back in time to AC 2, around Ezio's first years as an assassin -- let's say the 2 (i think) years that he trains with Mario. Desmond is like "fuck it" and decides to chill with the occasional murder of an unsuspecting asshole of a templar. I am partial to the idea that he's a lone mercenary of some kind, touring italia and the more unknown for him places (Milan, Verona, Naples) and successfully NOT involving himself with any assassins.
And then Ezio actually starts his career and is getting a lil' famous, mostly among his own allies and enemies.
It starts out simple, a little predictably even: Desmond is back in Florence, fully confident that Ezio is not around at the moment, hiding his familiar features from courtesans, thieves and mercenaries alike, when some guards spot him, maybe bump into him and fly into a rage. While chasing him, Desmond sees that a certain someone hasn't taken off his wanted posters or paid off the town criers.
Some mercenaries in Forli mistake him for Ezio, some courtesans give him winks with different meanings and even the thieves stay mostly away.
Things are not so fun for Ezio. A weapons merchant visits Monteriggioni and greets Ezio like an old friend, Caterina's friend -- a politician from Milan -- quietly thanks him for some shady favour, and a few foreign women starting work at la Rosa Colta are especially friendly and attached. What the hell.
As the years go on, it gets worse. Desmond doesn't get anything from the assassin tombs, but Ezio can tell that someone has been there before him and RECENTLY. A few of Mario's new men that haven't had a good look at the young master yet have been spilling nice, but untrue rumours. And when Desmond decides to finally visit Venice about a month before Ezio? Well, Rosa reaching for the wrong pocket wasn't noticeable, Antonio commenting on how "slow he's gotten" is weird, but Leonardo asking him how he is liking a book that he's never even heard of? That does it.
"Seriously, amico, what the hell?!"
"Um, Ezio, did I speak out of turn or something?"
"No, merely being the hundreth person making me worry about either a secret twin or a worrying sleep-walking habit!!"
It doesn't help that Desmond paid Rome a visit over a decade before Ezio.
It's not his fault he didn't really recognize the bratty but impressive teenager he spent an afternoon teaching how to properly climb. It's not his fault that he had an insignificant but profound impact on the kid, and it's not his fault that Machiavelli is acting weirdly respectful and familiar towards Ezio!!
"But, Noodly, horror of horrors, wouldn't they know his name, and thus GASP Ezio would have already heard Desmond's name in a confusing context at least once in his life!!" Why, yes. In comes Minerva as the final piece of the puzzle.
Not too long after Monteriggioni's fall, Ezio spots a tall, strong, fast, tan-skinned man with a scarred lip eyeing a blacksmith's shop Ezio was about to invest in. Ezio, in all his ashy and bloody glory, fresh from burning a Borgia tower goes up to this stranger, grabs him by the shoulders and asks him in the most calm voice on earth:
"What is your name?"
