A collection of images showing distinct separation of two or more worlds or realities, using a medium of some sort to show a great contrast.
Mediums used can include, but are not necessarily limited to, physical objects (i.e., mirrors or reflection in water), stylistic separation (fade in/out, flipping), or artistic effects.
There must be a clear distinction of separation or contrast, but both worlds must be related in a way (no cross-overs).
This kind of method can often be used to show "what-if" scenarios, as contrast between different identities and/or personalities, contrast between present and past (or future), as well as many different purposes.
dual_persona tag does not qualify, unless a medium is clearly used.
Most often, characters are used as subjects, but backgrounds or worlds can qualify as long as there are mediums used.