I think I may have leapt to an incorrect assumption. This is for TV or film, rather than the stage?
It's not the screenwriter's call how the effect of twins (or clones, or invisible people, or characters who turn into monsters) is created. That's up to whoever does their production design and effects. Most likely twins would be one actor playing both characters, with body doubles seen from the back or their faces blocked when they're in the same scene--but you wouldn't write that in, because whatever problems that presents in filming are not yours to solve. Your whole job is to tell the story that includes the twins.
You'd write it as two individuals, which is what twins are, after all. The first time each is introduced, their brief character description should note they're identical twins as well as note any ways in which their appearance is radically different.
INT. HALLWAY - SUBURBAN HOUSE - DAY
KARYN, dressed in pink and ruffles, pauses at the door to her identical twin sister's room and pushes it open. Everything is black or deep purple: draped lace, skeletons, skulls, and coffins. Her sister MARYN, dressed in full-black goth, closes the door in Karyn's face without a word.
Make sense? It would probably serve you well to read screenplays--what they filmed from, not transcripts--for movies or TV shows with twins played by a single person, especially recent ones that were not scripted by the director-producer.
Maryn, who has no twin