Correct for grammar-both of whom?

The phrase "both of whom" is grammatically correct. It is used to refer to two people or things that have been previously mentioned.

For example:

* "I met two friends at the party, both of whom I hadn't seen in years."

* "The company hired two new employees, both of whom have impressive resumes."

If you are unsure about how to use "both of whom," you can try replacing it with "they both" or "both of them" to see if the sentence still makes sense.