In an article about a snide memo from the Obama camp (regarding Hillary Clinton's financial interests in India) is an interesting use of the plural possessive.
The AP article explains that the memo
referred to Bill and Hillary Clintons' [sic] investments in India; her fundraising among Indian-Americans; and the former president's $300,000 in speech fees from Cisco, a company that has moved U.S. jobs to India.
So apparently the surname now has to agree in number with compounded personal names.
- Bill Clinton
- Hillary Clinton
- Bill and Hillary Clintons
Benefit of the doubt disclaimer: It could be a simple cut-and-paste/editing error. It might have originally read "referred to the Clintons' involvement" and when the names "Bill and Hillary" were added they writer/editor didn't catch the change. But that's so boring an explanation that it's probably true.