Interesante pregunta. Los dialectos en los que no se usa vosotros evitan este dilema, usando siempre ustedes para el plural de tú / vos y usted.
No sé si tiene la RAE alguna prescripción para estas circunstancias, pero en el habla normal de mucha gente de España también hay cierta 'asimetría', y es normal usar vosotros para el plural de ambos tú y usted:
... vosotros is, in fact, the only productive second-person plural form for many Spaniards, for whom it serves as the plural of both tú and usted. Despite the universally expressed view that there exists symmetry in the Castilian system, such that vosotros is the plural of tú and ustedes the plural of usted, we show that there is in fact widespread asymmetry from singular to plural, i.e., a person might be addressed as usted in the singular while at the same time forming part of a group that is addressed as vosotros by the same speaker.
...
Given that a particular instantiation of the second-person plural can embody multiple such individual relationships, we first sought to determine if one V interlocutor was sufficient to trigger ustedes in the plural (much as in Spanish one male member of a group renders a plural adjective grammatically masculine). It was not. It turns out, in fact, that many of our interviewees admitted to using vosotros (and/or its corresponding morphology) in one scenario or another with a group of interlocutors who would all have been addressed individually as usted.
