dual persona
Two different versions of the same character appearing together in the same scene. Example: Saber & Saber Alter.
If the picture depicts them as two distinct characters in the same scene, don't use solo, 1girl or 1boy with this tag. Exceptions are split theme, multiple views, creature and personification and similar themes.
When the character is meeting their older or younger self, also tag age comparison if the personas have no interactions, or time paradox if the personas have interactions other than posing for the camera.
If the instances of the character are identical, use clone instead. If more than two personae are in one image, use multiple persona instead.