Honestly I wouldn't be mad at a the more likely sitcom vibe the show would adapt if both of them could see the ghosts. I think it would be equally as funny and not lose much at all really given that it's meant to be a u.s version of a BBC show it would set them apart and bring a more familiar feel to the show. BBC shows with supernatural things seem to quite often play on the "only one person can perceive this phenomena" whereas U.S. shows tend to be more filled with multiple storylines while the main A line is going and then a summary near the end to bring all the characters experiences closer together. If jay could also see the ghosts it would open up more chances for both Jay and Sam to have screen time with the ghosts and make their connections deeper/more meaningful.