Complex sentence: The First Officer, whose presumably functional AI was now showing a left bank, said that his AI was also toppled, but there is some belief that the Captain mistakingly took this to mean that both primary AIs were indicating a right bank.
Split into simpler sentences: The First Officer, whose presumably functional ADI was now showing a left bank, said that his ADI was also toppled. It is believed that the Captain mistakingly took this to mean that both ADIs were indicating a right bank.