The strange thing is that Maxim is only able to overcome Evil Maxim in castle B (the castle associated with Evil Maxim) and not the castle that is associated with himself. You would expect it to be the reverse.
The thing is there is a sequence of events with with HoD which plays out between the 2 castles. Where as with Sotn it was linear, Normal Castle>Inverted Castle.
HoD is more complex, Death mentioned the two castles were supposed to eventually merge and they were 2 "layers" or planes of the same castle. Lydia's sacrifice - which never ended up happening in game - was supposed to resurrect Dracula. It's my contention that at this instance (which occurs in the throneroom) the castle becomes "Castlevania" rather than 'formerly Castle Maxim' and this point there is no turning back for Maxim, as the influence of the remains consumes him once the sacrifice occurs. (The notion of the castle potentially becoming CV was touched upon by IGA in an interview from memory).
Dracula's resurrection never happens so the Castles remain unmerged. The next part is more technical; Maxim(A) in Castle A is the original component of Maxim, Maxim(B) in Castle B is the 'influenced' (by the remains; morally weak, evil, etc) component of Maxim. If you fight the original component only, you'd kill the whole Maxim(A) and Maxim(B) also ceases to exist. As opposed to killing Maxim(B) where you fight the 'influenced' portion of Maxim... (However you can still kill Maxim(B) in his entirety unless Lydie's bracelet is worn.)
If what Death says holds water, what happens in one layer effects the other layer, which is what I've explained above. If Maxim(A) dies, his will and both Castles go with it. The reason they've only made Maxim(B) react to Lydie's bracelet is twofold imo:
1) Evil/ 'influenced' (by Dracula, his remains or whatnot) individuals often react unfavourably to "good/pure things" or triggers that resemble them. The way this is handled CV games is by equipping something like a bracelet.
The bracelet resonating with Maxim is what finally purges Maxim(B) out of him, and then the remains resonate - in Castle B only, being associated with chaos, or whatever one wishes to call it.
I do believe if Juste had lost this battle, the castles would still merge, but with Maxim as the host body for Dracula: notice how the remains turn into Dracula Wraith whose second form is (you guessed it) a cluster of actual remains.. If Maxim(B) had won, Dracula would return the way he did with Isaac in COD imo. This may also have happened in Castle A, but it makes little difference if the Castles then merge.
My take:
Why this only happens in Castle B is because B imo is the "Chaotic" or more demonic side of the castle which is why it's associated with Maxim(B); 2 Maxims with the same will except one is still clinging to good and retaining his humanity(A), while the other(B) is trying to resurrect Dracula, has extraordinary powers (compared to the original, which is the one you play as in Maxim mode) and has almost succumb to the influence of the remains.
Because the theme of the story re: Belmonts is a hero defeating evil/ Dracula, the shit only goes down in Castle B.
I could go more into this but I'm on a mobile device, so I'll try to summarise.. Let's say I'm leaning towards Castle B being the side from which "chaos" (perhaps via the influence of the remains, as we've seen Barlowe undergo in OOE) flows.. Therefore the Maxims(A+B) are the same physical being, fought at the same instance under slightly different circumstances. Fighting Maxim(B) would be like fighting Richter with the Holy Glasses, while fighting Maxim(A) would be like fighting Richter without them.
2) There's a logistical quality whereby the developers provide a number of endings. For example, there's no reason stated explicitly as to why Richter Belmont dying causes the Castle to collapse(or both Castles) in Sotn. (Shaft wanted Dracula's resurrection, he was using Richter to ward off other Vampires [and most likely open the seal in the 'infinite corridor' as per plottwist's theory] but in reality Alucard killing Richter should still allow him to progress, but it doesn't.)
Pertaining to my take in (1):
The purely straightforward way to see this outcome is that the Maxim(A or B) battle in the final room is when the 2 castles are starting to merge or overlap (like an eclipse); Either Maxim winning causes the merge, either Maxim losing stops the merge, but if good Maxim(A) loses then Maxim is lost, if 'influenced' Maxim(B) loses then the real ending happens.
I do believe with CV it can be that simple as the gameplay and design will come first with the story to support the themes/ ideas of the gameplay.