There is no particular reason dual_persona is different from 3+ persona (multiple_persona). Many other "multiple" tags, like multiple_girls, multiple_views, all include the case of two. (Well, multiple_arms is 3+, but for obvious reasons.) Should dual_persona be a special case of multiple_persona instead of being disjoint with it?
A big reason to make the suggested change is so that age_comparison can then imply multiple_persona.
Updated