I was thinking of this reading the Remus and Romulus myth. What if Remus, had gotten away, and made his own city across the Tiber, and called it Rema, what could happen? I am already writing a timeline. ASB or not?
I am thinking of, Rome forming the Roman Empire north of the Tiber, with Rema forming the Reman Empire/Republic
Probably not going to happen, but interesting idea. If both were fairly equal and huge cities destined to create as big of empires as you say, one is going to have to triumph over the other unless they're Milan - Taranto apart as best case scenario for you. Rome and Carthage were seperated by a few hundred miles of sea and they were too close to each other for them to coexist with their own seperate empires. Two huge empires (both based on the Tiber?) are way too close for one to be conquering Britain and the other Arabia without one conquering the other, IMO.