They’re not. “They’re named Trevors” is a correct but unbelievably clunky sentence no one would actually say. They are both named Trevor. So since there is two of them saying “They’re named Trevors” is factually correct. As there are two people named Trevor there. Hence two named Trevors.

I was just being stupid and having fun with the English language. Did get doubly funny when I created Trevi as the plural and then there was some daft point about the plural of Liverpool.