Complex sentence: The Romans eventually convince Getafix to make an antidote, and Getafix does make it-but at the same time he also prepares a small quantity of the real magic potion for Asterix to drink.
Split into simpler sentences: The Romans eventually convince Getafix to make an antidote, and Getafix does make it. At the same time, he also prepares a small quantity of the real magic potion for Asterix to drink.