Even then, considering that Buu once Gotenks separated felt in need to immediately absorb Gohan as a contingency plan he already had, it's far less possible that he would dare to fight a fusion between Goku and Gohan. This is also taking into consideration that he dared to challenge SSJ Vegetto being Gohan Buu.
I think things would go down this way: Buu would try to absorb Gokhan but he would use his protective barrier as Vegetto did and would save Piccolo, Goten and Trunks, then Super Buu would turn back to normal and considering that Gohan alone is enough to kill him, Gokhan would be beyond overkill, but it's in character for Gohan to humiliate him as he did with Cell, then Super Buu would try to destroy himself, but, if Gokhan is serious, then he would immediately kill him.
Then, maybe they can use the Dragon Balls or the Namek Dragon Balls to separate them (the first once are somewhat unlikely as it didn't turn 17 and 18 into humans, while the other ones, got also stronger after Freeza incident and separated Kibito Kaioshin in Super).
Sad thing is that Fat Buu would be killed, wonder if they'll alow Vegeta to return or if his last appearance would have been against Fat Buu.