What are the odds of that preventing pregnancy? Probably VERY slim, your body has to have time to get the B/C in your system, that can take up to a month. And it's pointless to take BC for two days then stop, if you're having sex, birth control should be an everyday thing, not a once in a while thing.
Dont quite see why you would do that. It'd be pointless. If you dont want a baby and you are sexually active you need to sort out long term birth control.