There's only one Dweller in Darkness. He originated in the Previous Universe like Galan.
He has no counter-parts, so when he appears on panel, it's him, the only him there is.
The problem with Adv. of the X-Men is that the book doesn't make it clear where/when it's situated.
It does tells us it's an alternate reality, but this is misleading, cause it is, but in a special way.
You see, Adv. of the X-Men reality, is supposed to represent
the previous universe that Galan came from, the very First reality ever.
Then in pure idiotic fashion, they jam the LT and Brothers (DC too wtf)
into the story,
in order to try and justify the creation of the New Marvel universe and DC, again wtf.
Spiraling further into absurdity,
they stamp the initiation of DC's creation along with Marvel's onto the M'Kraan Crystal, wtf.
So, the Crystal implodes both verses, then the LT, with Spectre's consent,
fashions the Brothers so that they (Brothers) can create/embody both verses.
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The story basically, was an attempt at retconning the creation story of both companies.
But they failed miserably because it ended up creating contradictions/paradoxes
that are beyond reconciliation even with the loopholes.
So, technically yes, as originally intended by Macchio & Carlin
the LT fashioned both DC and Marvel, and Spectre was LT's equal.
But, since this idealism crashes all thought and reason regarding DC ...
... let's just say, the LT fashioned two separate totalities.
One Marvel, the other? A mystery = to Marvel.