The current wiki for dual persona:
Two different versions of the same character appearing together in the same image. Example: Saber & Saber Alter.
Each persona counts as a separate character, so don't use solo, 1girl or 1boy with this tag.
Do not use this tag for reference sheet, variations, multiple views, or other depictions of the same character from multiple points of view.
If the instances of the character are identical, use clone instead. If a character is meeting their older or younger self, see time paradox. If more than two personae are in one image, see multiple persona.
This should not be strictly exclusive from 1girl/solo. Saber and Saber Alter are separate individuals and understandably the 89 dual_persona saber saber_alter posts are tagged multiple girls. They are separate characters in the same setting, no confusion here. One, post #3649023 is tagged 1girl solo. Debateable: you might argue that the sword is reflecting the viewer, but the artistic intent is to overlay a part of Saber with Saber Alter (matching).
Then we have Ame-chan and Chouzetsusaikawa Tenshi-chan. The latter is Ame's appearance when she streams; they are the same person and it thematically shows in some posts like post #5477055, post #5087544, and post #5085920 tagged 1girl solo. A small picture is held over them. Imagine a normal post that contains a picture of a totally different character; the characters are tagged and the overall post is solo.
Recently in chat someone asked about post #5626965 and someone pointed out "Each persona counts as a separate character" then 2girls was added. I disagreed, which prompted my writing of this thread. There are more pieces this time but they still overlap and match the body's pose/silhouette. The pictures are not portals to a second body nor does the second character act independently.
These are not cross-section, revealing layer, x-ray, or reverse x-ray. What are they? post #4773567 (not actually dual persona) is even more superimposed. The rationale behind 1girl is we're seeing specifically one body, not one in front of the other.