I guess this wasn’t a problem for the Medusa of old because there was only one of her. Medusa wasn’t immune to the sight of herself, hence the ol’ mirror trick. But the snakes in her hair must have been able to look at each other all right, otherwise they would have been caught for sure at some point. Maybe isn’t because snakes aren’t very visual animals. It’s possible they would have turned to stone if they sniffed each other with their tongues or something, so they probably had to keep them in their mouths. It would make it hard to eat, but probably Medusa didn’t feed them much anyway, since they don’t have back halves and she’d just end up with her head filled with dead mice.
UPDATE: A couple of people have written in to let me know that Medusa had sisters, Stheno and Euryale. There’s really no excuse for me not knowing that, since that’s how it is in One Piece too. However, Medusa was the only one who could be killed. Also, she didn’t turn to stone from seeing her own reflection, although I’m sure I saw that somewhere (basilisks, maybe?) — she just got beheaded. Parts of my mythological knowledge come from sort of dubious sources, so the heads-up is appreciated!