There were often times when the Byzantine Empire had two or more emperors. During these times, how did the emperors interact with each other and how did others interact with them? In addition, how was power divided in these scenarios?
I'm interested in any and all answers anyone can provide, but this question originally comes after thinking about the story of Basil I and Michael III, so if anyone could provide information about them in particular I'd be very happy.
I've asked this question a few times before and never received an answer, so here's hoping someone can answer this time!
Upon reviewing Byzantine history, you find that most co-Imperial relationships were very lop-sided in that one Emperor held almost all of the power, while the other acted as a figurehead. This is likely because if two people held equal shares of power (especially in the Roman or Byzantine Empire), it would almost always erupt into civil war. Therefore, the political ambitions and intrigue of the Empire almost universally didn't allow for two people on equal grounds to peacefully coexist. What you are left with is a tenuous alliance between two powers vying for the throne (generally great families) - the co-Emperorship was a compromise to prevent instability and revolts.
Some great examples are:
Romanos I Lekapenos and Constantine VII Porphyrogennitos: When Leo VI died in AD 912, he left his child son Constantine as the heir to the throne with his fourth wife Zoe Karbonopsina as regent. Romanos, a famed general at the time, saw the opportunity of a weakened government and seized control of the throne. Since Constantine was young, Romanos was able to exert a huge amount of influence over him, and acted as the grand puppetmaster at court and abroad. It was only 30 years later that Romanos was removed from power for good, allowing Constantine to reign on his own until his death in AD 959.
John I Tzimiskes and Basil II: Basil was still a young man when Tzimiskes murdered Nikephoros II to take control of the throne. Since the Macedonian line had ruled for nearly 200 years by this time, it would have been political suicide for Tzimiskes to simply cast the two heirs of Romanos II (son of Constantine VII) aside. Therefore, while Basil and Michael were the rightful heirs, Tzimiskes held almost universal control of the military, as well as the adoration of the people for his triumphs, and so his position could not be challenged, but again, he had to honor the long line of the Macedonians.
Romanos IV Diogenes and Andronikos Doukas: Romanos held all of the power, and kept Andronikos Doukas "hostage" by preventing him from gaining followers. Andronikos only existed as co-Emperor to appease the Doukas family, who held the Imperial throne before Romanos (Constantine X ruled until his death in 1067, when Romanos became the lover of the Empress, and was crowned himself). At the Battle of Manzikert, it is sometimes said that Andronikos purposely did everything he could to sabotage the battle plan, causing the situation which allowed for the Emperor Romanos to be surrounded and captured.
During the Komnenian period, titles were created to appease disgruntled family members. Alexios I Komnenos needed to be able to prevent revolts in an Empire that had very nearly buckled because of its own internal struggles. Therefore, lofty titles such as panhypersebastos (meaning "exalted beyond all others") and sebastokrator (meaning "exalted ruler") were given to relatives so that they would support the centralized power of the Komnenos family without revolting. This was because a title was seen as an award that carried a certain amount of grandeur and weight. In reality though, these titles were simply meant to appease and flatter. The people who carried them held little real power. This is essentially an extension of how the co-Emperorship worked throughout Byzantine history.