As far as I know, we lack a tag for this concept. The closest way to search for it is (~dual_persona ~multiple_persona) (~yuri ~yaoi ~hetero ~bisexual_female ~bisexual_male ~futa_with_female ~futa_with_male ~futa_with_futa ~futa_with_cuntboy ~cuntboy_with_male ~cuntboy_with_female ~cuntboy_with_cuntboy) -selfcest which isn't ideal (it's long, dual/multiple persona often get mistagged for multiple_views, selfcest is sometimes untagged).
I preliminarily made favgroup #25904, but:
1) It's still a massive work in progress.
2) I added those posts to the favgroup based on what the posts looked like from this search: dual_persona ~hetero ~yuri ~yaoi -selfcest status:all limit:200 - I wouldn't be surprised if some posts in that search were mistagged, and characters depicted are actually twins or lookalikes... but I can't know every copyright.
3) I don't know what the actual tag should be named. The favgroup name is only a placeholder, and doesn't account for posts like post #6617816.
4) I don't know if this tag should be limited to romantic/sexual situations, or should platonic ones like post #5183773 be covered too (or maybe a different tag could be made for it?).
Is this a viable concept to create a tag for?
Updated