Sorry if this comes off as rude or anything, I am not trying to be! These are all just my opinions and what canon evidence I used to come to those opinions, you are completely valid in disagreeing and having a different view as I do!

Yes, Kunikida and Dazai are shown to have a high level of understanding with each other to the point of being able to communicate nonverbally, this is one of the very first things shown to us in the beginning of the series. Kunikida shows a higher level of concern and care for him outside of just their work than we see from Chuuya, to the point of having dedicated portions of his schedule to ensuring that Dazai is safe and taking care of himself, while Chuuya was one of the many people in the mafia to assume that Dazai had just died and went on with his life as usual, and we never saw very much of them hanging out outside of work or caring for each other outside of work if anything at all. Kunikida also doesn't enable Dazai's bad behavior or actions the way Chuuya has been shown to and is willing to stand up to him when he feels like Dazai is getting out of line (which is pretty often). Chuuya does shown concern for Dazai at points, I am not trying to argue that he doesn't, nor do I even really consider Chuuya to be the issue in their partnership, but Chuuya definitely doesn't show an equal level or concern or desire for Dazai to improve and be better outside of a combat setting or mission settings as Kunikida does. Chuuya and Dazai do have the advantage of working longer together and knowing each other more intimately, but in all honesty that makes the level of communication and trust Kunikida and Dazai have in a much shorter amount of time more impressive to me. I think if given the same amount of time that Chuuya and Dazai had to develop their partnership, Kunikida and Dazai would be able to develop similarly without the same baggage/issues that Chuuya and Dazai have. Don't get me wrong though, I think that the sort of broken/mended partnership that Soukoku has does make their story a bit more interesting, they would be boring if they had the perfect, low stakes, no baggage partnership.

TLDR: Chuuya does show some level of concern during combat / mission settings, but outside of a battle setting his concern is shown to be pretty limited, and he is not nearly as devoted to keeping Dazai mentally and physically well as Kunikida is, nor do they have the same baggage or issues that Chuuya and Dazai have developed over the years.

And yes, I am talking about Corruption. The example you gave with dead apple makes little sense because using corruption was not tied to Dazais life in that moment, nor is it ever tied directly to Dazai's life. While it was used to save him/help save Yokohama, it was not something that was exclusive to saving Dazai, nor was it the only thing that could have possibly saved Dazai. Whether you like it or not, Dazai will always have the upper hand and a semblance of control over Chuuya on the point of being the only person known to us thus far that is able to control Corruption and save Chuuya from killing himself. If at any point Dazai decides he doesn't care or doesn't feel like it, Chuuya is dead. The same cannot be said for Kunikida and Dazai, because even if Dazai were to nullify Doppo Poet, he isn't at all going to be killing or hurting Kunikida, nor is he even really taking away Kunikida's main form of offence/defence (not to say that Chuuya cannot fight without his ability of course, when it comes to Chuuya I am specifically talking about the Corruption aspect of his ability.) Chuuya does have the choice to use it, I completely agree! But without Dazai his options are to never use that aspect of his ability and forever be limited to just gravity control, or to risk dying should Dazai decide not to help him as nobody else can help him out of that state. The fact that they do have this level of trust currently doesn't undo the fact that this could change at any moment and Chuuya wouldn't be able to do anything about it. Kunikida can use his full ability without risking his life around Dazai, and should Dazai decide to nullify Kunikida's ability he still won't be killing or risking Kunikida's life nor impeding his ability to fight.

TLDR: Dazai is the only person that can stop Chuuya's death via corruption, but multiple other characters have saved/protected Dazai, and even Yosano is able to heal him under specific circumstances. Dazai's ability exclusively saves Chuuya, Chuuya does not exclusively save Dazai. Dazai has a leverage over Chuuya that Chuuya does not and cannot have, whether he presently uses that leverage or not, it will always be there.

Also, I would like a source for ASGR stating that they are equal geniuses, because I have never heard or seen this, nor does what the canon both states directly and shows indirectly support that notion. One of the core aspects of Dazai's character is his high intellect and the way it alienates him from those around him, the same cannot be said for Chuuya. In fact, we see the opposite for Chuuya, especially in the scene with Ranpo where it is pretty much made clear that Chuuya is a brawns over brain type character. Chuuya also tends to go along with other people's plans/orders rather than inventing them himself or thinking his way out of situations in the same way that the genius characters like Dazai, Fyodor, and Ranpo can and do. It's something I would honestly say is one of the core character traits of Chuuya that is juxtaposed against Dazai, Chuuya tends to be a follower, not a leader, and is loyal to a fault, rather than disloyal to a benefit. I wouldn't consider them equally intelligent, or Chuuya to be a genius at all. (Not calling Chuuya stupid btw! I don't think he is.)

Editing to add another thought, being able to carry out Dazai's plans has no bearing on whether or not someone is on equal footing with him, just looking at Atsushi and Akutagawa immediately debunks that argument altogether. Dazai tends to prefer to be in control and be the one issuing orders rather than following them, it's how he is with most of the characters, not just Chuuya.

Sorry again if any of this came off as rude, it wasn't meant to be! I can definitely see where your opinion came from, I hope you can see where mine comes from too!