Reverencial voseo vs. Vuestra merced
 

Which sounds more formal? What's (or more accurately what was) the perceived difference between them?

e.g.
Vos sois una persona inteligente.
and
Vuestra merced es una persona inteligente.

And how would one address two people that one addressed with reverencial voseo? Not vosotros, obviously, since that was informal, right?

vuestras mercedes (or "vuesas mercedes"), five or six centuries ago

ustedes, nowadays

Basically, "vos sois" is addressing one sole person by using plural, a polite form that is today common in other languages. "vuestra merced es" is addressing a person in third person, a polite form that is today common in other languages and Spanish (usted, ustedes).

Today, both expressions doesn't sound anything but dated, and the degree of formality is just in the ear of the "beholder".

The hierarchy between "vos", "vuestra merced", "vuestra señoría", "vuestra excelencia" and pronouns derived from these forms is unclear and it seems it depended highly on the region where they would be used. "Vos" could be respectful enough for some, while others would consider it disrespectful.
The way they're used in antique texts has been kept as it is, but has rarely been emulated. However, when someone wants to make a fake atmosphere of archaic respect or for translating old texts, the usual treatment is "vos".

By the way, the plural of "vuestra merced" would be "vuestras mercedes".
And "vosotros" is only informal in modern Spanish in Spain.

So do you mean it used to be formal sounding in Spain? And that it sounds formal when people from other countries hear it?

No present tense for "when people from other places hear it". These words are not heard anymore as respectful formulas anywhere.

"Vos" and "vosotros" were used many centuries ago as ways to respectfully address people of a higher hierarchy. And of course "vosotros" is the natural plural.

The current uses of "vos" (as "tú") and "vosotros" (as a group of people to whom one addresses with "tú") have evolved in specific regions and have nothing to do anymore with the way they were used in the past.
In all Spanish speaking regions where "vos" or "vosotros" are never heard (even in their modern forms), we understand them exactly as they're used in their own regions.
If the context places those expressions in an archaic context, we'll obviously understand them as antique respect formulas.

"Vosotros" was never very polite. Not more or less polite than "nosotros". "Vuestras mercedes" was the polite form. I suppose the reason Spanish from Spain keep "vosotros" and "tú" is that "tuotros" doesn't make much sense.