(Geto is still alive in this au btw)

After Satoru died in the battle against Sukuna, Satoru was reborn as a cursed spirit, Suguru, his best friend since high school, he absorbed the curse and Satoru was rightfully his now. As a cursed spirit, Satoru was way stronger than he was when he was a human, but yes, even if Satoru was the strongest sorcerer, he still lost against Sukuna and died.

Now that Suguru absorbed the curse of Satoru, they can't leave each a second time, I mean..Suguru probably didn't want to leave Satoru alone the first time but he chose the path of being a curse user and a cult leader. But with Satoru like this now, they can stay together forever. Don't have to worry about leaving each other again unless Suguru was to die but Satoru isn't going to allow that, he was going to keep Suguru from dying. They're together forever.