[Spoilers] My story speculations

[Pony Flare]

Read a few things here adding my own thoughts.
I believe that, while the ending is clearly purely Renie's world, at the beginning, this is a shared word between Renie and Noah.
While many things from The Whispered World are kind of "twisted" by Renie's interpretations of Noah's story of Sadwick, the first part while played as Noah clearly shows items damaged but otherwise exactly as he would have remembered them from his first time in this world. It's once they exit the world worm that Renie's influence on the world truly begins. It's like that worm ate his old world, and this is a new one.

Noah is fully himself until the scene where he takes the jester's cap from Renie after discovering that she's the queen of this world, after which he lets the Sadwick persona take over with the mistaken belief that he's not actually Noah.
He remains something of a mix between the two until Shana freezes him, then revives him with pure Sadwick. It is only from the point where he is frozen that this version of the Silence world truly belongs exclusively to Renie.
He gets too much time to himself in ways that just don't seem like they could come purely from Renie's mind until that point.

Not going to add anything about the endings, as that's been discussed to death already.

[Kelrycor]

I think Silence is completely and totally Renies Silence. And the real Noah was never there. And I will gladly give you an explanation why it is so in my personal point of view:

Silence is created by the mind and the memories of the person in coma or actually is dying - this we know from both games. In the sequel it is made also more clear that the person who created the world is the King or Queen of Silence. In Noah's Silence, in the prequel, there was a King (who actually is Noah's true self in the mirror), for Renie's Silence it was the false Queen, reflecting her true self as an "antagonist" (to quote her: "I am freezing to death!" as the real Renie is lying in the snow). A combined Silence from Noah and Renie in the second part is not very likely, because the false Queen has no "false King" on her side, not even in the beginning it is told that there is also a king.

Plus, you don't see any wounds in neither ending on Noah at all. But Renie has, directly in her face when waking up - so we can guess she was hit on the head very hard with a shrapnel/stone because of the bomb explosions, which is most likely the cause why she nearly died or being briefly comatose. A very nice detail is that the wound is shaped like the broken part of the mirror and is also seen on the face of the false Queen - clearifying once more that it is her mirror, not anyone elses (see at 3:42): https://www.youtube.com/watch?v=KF4DYKRAb0o

Remember the credits of the prequel: the Chaski was the postman, Bando was the binman, Ralv and Yngo were just stones painted a face on while Noah played as a kid, and the combination of his doll and dog resulted in the caterpillar Spot. So persons and things, and the story his father read to him, were indeed reflected in the world they created, the Whispered World (aka Silence).
Also remember that it is said that Noah told Renie the stories of Sadwick as a goodnight-story in the sequel. So she knew not only Sadwick, but all the other characters like Spot and the two stones, and all what happened back when Noah lived through it. That is why a lot of persons and things we would consider being a part of Noahs Silence appear in her personal Silence too. Renie has a strong fantasy vision as a child, and her fictional world was influenced hugely by Noah. She was adopting a lot of things all along - not on purpose though, I guess.

The rest is just a part of story telling. On the one side the developers wanted us to think it is again Noah who is in Silence, on the other hand Renie is remembering Noah as Noah in the real world, but the more she stays in her Silence, she wants him to be like Sadwick. This is also pointed out in a specific scene they both have a small fight. That is why he changes to Sadwick-Noah in the process step by step. Why? Renie is the rightful Queen to Silence, she has the power to alter this "detail" all along - though she is not aware she can.

We know that Noah in the real world didn't die whatever fate Renie has chosen. But if Noah became Sadwick-Noah, and would share the world of Silence in any way with Renie, his transformation would be a sign to his resignation to Silence. And from what we know if you give in to Silence, you don't return to the real world... and die. As we know this didn't happen on both endings, Noah was probably all the time just shortly unconscious and never comatose nor on the brink of death in the real world.

All these considerations are making it quite clear that this times Silence Renies solitary Silence all the way.

[Fantasma]

I agree with the OP. I think that in the beginning of the game, we saw Noah's Silence which was destroyed after breaking the mirror and that was disappearing in a symbolic way in the stomach of the giant worm.

The mouth of the worm is the connection between the Silence world of the two brothers but Noah has no way to come back as his mirror is not there anymore. So both brothers are in the world of Silence of Renie. As the time passes, Noah's personality and soul is changing to Sadwick (due to the effect of Renie's world, it is what she wishes). It is also strange that you play with both actively in separate ways if that would be the story of Renie alone..

Why do I think that? Because of the part of the Oracle. First of all, Renie has the bourden of his brother soul, we can see that when she is transformed into a statue, holding the head of one of the "previous gods" which is similar to him and that breaks when Renie is freed.

Second thing that makes me think this, is when speaking to Shana. She says that the fragment of the mirror that they have is a part of the mirror that Sadwick destroyed (so it is the only part left of "Noah's world" which connects both worlds and that represents the leftovers inside that giant worm). The next sentence of Shana says:

"With it (mirrors' piece), the mirror will be restored again. The land will heal, the False Queen will vanish and with her the Seekers...; and lost souls will find their way home"

What happens if you choose to mend the mirror? The mirror is restored, the land heals, the false Queen vanishes (Renie becomes the real Queen as it is mentioned while it happens), the Seekers too (we see the scene where the Seekers of the city are freed with the ray of lights) and... lost souls (Noah) will find their way home (wakes up). It was also strange to me that when you mend the mirror, the false Queen says that "her brother will finally find peace", it looks like his Sadwick part will also stay in the Silence world, separating the clouds (Noah) from the rain that falls into the sea (Sadwick).

If you break the mirror, Renie wakes up and the brother does not respond, yes, he moves the hand, but that means that he is alive but not necessarily conscious, he may be in comma. Renie realizes this and she starts to tell him a story after understanding what happened, probably trying to find a way for Noah to come back which is not possible as the whole mirror of Noah was destroyed in the end.

At least that makes more sense to me than thinking that the brother always lives and you choose between dying or not, the false Queen has no real reason to lie to herself like in the first game, Noah did not lie to Sadwick. And now, all the hints given in the game would make complete sense with the "mend the mirror" option.

Greetings