When did they express fear of two Yonko fighting each other?
i can only point 1 thing , cause i just remind of 1 thing , obviously , which was when shanks was confronting kaido...
Actually they were afraid of shanks meeting wb, and that's what they showed fear of, that they might form an alliance. Op you might be confusing it with shanks interfering kaido/wb.
Probably because of collateral damage. When Akainu and Aokiji fought they permanently changed an islands weather, however this fight was more organized and they probably kept the fight within Punk Hazard to uphold public safety. There is no telling what might happen when two Yonko (and their crews) fight unrestricted. Especially if there are people of high priority near the vicinity (like a celestial dragon perhaps).
The victorious Yonko will be able to obtain all the land the other Yonko administers. Thus said Yonko will have greater influence and thus be a greater threat to the WG. There's also potential for one Yonko to recruit many of the surviving pirates of another Yonko. Said Yonko will have a higher chance of defeating the other Yonko now that he has gained an advantage, since priorly they all fought equally. Thus this Yonko can become Pirate King and eventually make all pirates go completely berserk like Gol D. Rogers did.
i see where you're getting at , but i disagree.
But, were they actually scared of the concept of one Yonko fighting another?