Something to start things off that no one else seems to have realized;
Kosoter was very likely the other decapitant.
Lol, some readers here have certainly argued that Kosoter might have been a Decipitant at least since Ciphrang-Malowebi's creation in TGO, if not before, as far as I remember.
Mimara would settle the question for me, if we knew she saw Ciphrang under the Eye as differing from just Damned people.
For instance, merchant quoted the portion of TUC where Mimara sees Serwa trying to save them and Mimara labels her a Ciphrang and this differs from her "demon" description regarding Kosoter and the Skin Eaters (where demon might just be a descriptive word). Which, by that argument, would make Serwa a Decapitant (personally I'll err on Mimara being unable to distinguish actual Ciphrang-Characters, or then authorial error should she, say, later recognize a Ciphrang-Character as a Ciphrang head on a body with the Eye).
I think, based on some of FB's comments regarding the
Atrocity Tales, the cipher of the Decapitants is that Kellhus has possibly created a number of Ciphrang-Characters, up to but not limited to Himself and Malowebi.