If I'm not mistaken, currently the preferable practice is to prescribe 1.5 mg of postinor as a single dose (2 x 750 μg), but you can still take the second 750 μg tablet anytime within 12 hours after the first pill. The reason why the drug company decides to seperate the doses is because large doses of progesterone induces vomiting, and somehow many people have a taboo against the word 'side effects'. However it is safer in terms of contraception to take one large dose (oral emergency contraception works by having large dose of normal oral contraception), and the chances of missing the second tablet for whatever reasons are much lesser.

btw, docs don't work in pharmacy, the lady should be a pharmacist, either way getting annoyed for that instead of providing relevant explanation is not very professional...