I've heard that having two character names that start with the same letter is a "no no."The question isn't if they are two characters, but two MAIN characters. Marcus and Molly as the two main characters isn't bad, but Mike and Mark, Tim and Tom, Harry and Henry...not so much.
Should I consider a name change for my characters if one is a 30-something black man named Marcus and another is a 10-year old white child named Molly?
These are little things I tend to notice when I read a manuscript, and if I notice it, it's probably something I'd suggest be changed in revision notes I send. It's not a deal breaker at the submission stage cause it's so easily fixed.
I have a client who shall remain nameless who at one point sent me a manuscript with two characters named Tom. And then there was the other one with two countries named Smog and Smogg I think. Fortunately, said client has learned to accept editorial notes with a minimum of hair tearing and garment rending, although he does seem to post this a lot: