I am writing a short formal acknowledgement in a paper, and I would like to thank two people at the same time. The phrase I'm using is essentially:
Coffee was paid for by John and Anna, to both of whom I express my sincere gratitude.
I am not sure if the expression "to both of whom" is correct. Should it be "to whom both"? Maybe a small variation of the phrasing would work, but I can't think of any that is short and formal.
Coffee was paid for by John and Anna, to both of whom I express my sincere gratitude.
is the correct of the two sentences you pose.
both OED
With a plural pronoun (denoting the members of a pair). Immediately following (or in early use preceding) a subject or object pronoun. Also in both which. Cf. sense C. 3, and both of us, both of whom, etc., at sense
To use to whom both in your sentence:
Coffee was paid for by John and Anna, to whom both I and my companion express sincere gratitude.
It could not be “to whom both”. If the clause were not a relative clause, you would write something like “I express my sincere gratitude to both of them.” You can turn it into a relative clause by replacing “them” with the relative pronoun “whom”, which needs to be “fronted”—placed at the start of the clause. But if you wrote *”whom I express my sincere gratitude to both of,” the preposition “of” would be “stranded”—something that is possible in some sentences, but not I think here. So other things move with the relative pronoun (this is called “pied-piping”). If we just moved the “of”, we would get “of whom I express my sincere gratitude to both.” This sounds a bit better to me, but I think that it still sounds worse than the option you mentioned with pier-piping of the phrase “to both of whom”—“to both of whom I express my sincere gratitude.”
Pied-piping is fairly formal, but you said that the context of the sentence is supposed to be formal.
Drop the word both. Whom is singular and plural, just the same as you is.
“Coffee was paid for by John and Anna, to whom I expressed my sincere gratitude.”
