My interpretation: “their name is Trevor” is grammatically correct, but semantically means their collective name is Trevor, not that they’re both called Trevor.
“Their names are Trevor” is incomplete because the subject implies a plural while the object is singular. This could work if you were to list multiple names: “their names are Trevor and Trevor.”
My interpretation: “their name is Trevor” is grammatically correct, but semantically means their collective name is Trevor, not that they’re both called Trevor.
“Their names are Trevor” is incomplete because the subject implies a plural while the object is singular. This could work if you were to list multiple names: “their names are Trevor and Trevor.”