What is the plural possessive of Messrs Van Ness?

Because the plural noun is "Messrs." the possessive form is based on the singular part of the noun "Van Ness".

There are two accepted forms for possessive singular nouns ending with an s:

Add an apostrophe (') after the existing s at the end of the Word:

• Messrs. Van Ness'

Add an apostrophe s ('s) after the existing s at the end of the Word:

• Messrs. Van Ness's

Examples:

Both of the Messrs. Van Ness' passports are in order.

Both of the Messrs. Van Ness's passports are in order.