Throughout the movie Avengers: Infinity War there’s some relatable exchange between Thanos and Stark. Either they are talking to each other about each other or talking to others about each other.
At Doctor Strange’s sanctum where Banner is making everyone aware of what’s coming and Stark goes "Tell me his name again?"
At Ebony Maw’s spaceship while Tony and Strange are hatching a plan Tony says something on the lines of "I have been waiting for Thanos since 6 years since New York"
At Titan during the battle, Thanos identifies and calls out “ Mr Stark” to which Tony asks "You know me?" And Thanos responds "You’re not the only one cursed with knowledge"
To conclude, at the beginning Tony doesn’t know Thanos but later he says he’s been waiting for 6 years for this revenge and later Thanos recognises him. So do Thanos and Tony share parallel history and both are aware/unaware of each other?