So, in the middle 7th century, the Byzantine Emperor Constans II conducted the last 5 years of his administration from Syracuse, before being assassinated in his bathtub. Some feared that he might have been trying to move the capital west, but its likely that he was just trying to focus his efforts on holding onto the western territories the Byzantines still had.
So, lets posit this scenario: Constans is more successful in his campaigns in the west, taking the duchy of Benevento, solidifying the Byzantine hold on the southern 2/3 of Italy. He's even able to, for the time, hold back the Arab advance out of Egypt into Africa, through some fashion.
However, simultaneously, a usurper (maybe Saborios) is able to take Constantinople and declare himself Emperor. So, we have Constans in the West, with more general legitimacy and a sizable portion of the military, as well as control over the actual city of Rome, and Saborios in the East, with Constantinople and an alliance with the Arabs.
What happens next?
So, lets posit this scenario: Constans is more successful in his campaigns in the west, taking the duchy of Benevento, solidifying the Byzantine hold on the southern 2/3 of Italy. He's even able to, for the time, hold back the Arab advance out of Egypt into Africa, through some fashion.
However, simultaneously, a usurper (maybe Saborios) is able to take Constantinople and declare himself Emperor. So, we have Constans in the West, with more general legitimacy and a sizable portion of the military, as well as control over the actual city of Rome, and Saborios in the East, with Constantinople and an alliance with the Arabs.
What happens next?