In my opinion, if you are living under their roof, and they say no.. then you shouldn't go get another snake.
Convince them by proving you know everything about it's care(show them "neat stuff you found about how to heat the enclosure", and how you have the bills for them figured out). Make sure to keep the currant snake's cage PRISTINE. Offer to do something extra at the house in order to be able to keep a 2nd snake. Do you wash the dishes? The laundry? Clean the fridge? Something? I know doing a couple extra chores would go a long ways to convincing a lot of parents.
If nothing else, you can always wait until you move out on your own. Also... have you thought out what happens when you go off to college? Who will keep them then? Sneaking them into a no-pets dorm is not a solution. Show your future planning.
All sorts of ways to show you are dedicated to them. But end result will be it's their house, and if they say no, then you shouldn't go get antoher one.
Hope this helps.