There is no common word (though see below) for this relationship in English, but there is one in Spanish: _concuñado/a_. So William is your concuñado. This word is used a lot at some of our family gatherings.
Note that cuñado/a is Spanish for brother/sister-in-law.
However the terms _co-brother-in-law_ and _co-sister-in-law_ are sometimes used for this in English, but they are generally used when translating to English from a language that has a specific word for the relationship.