If guidance is used in conjuction with the Bit of Luck domain ability, does the +1 apply to both d20 rolls or just to the first one? I'm leaning toward applying it to both rolls, as they make up only one "single attack roll, saving throw, or skill check."
In general, "roll twice and take the better/worse result" means to roll twice with all of the same modifiers. This includes things like guidance, true strike, the one-time attack bonus from charging, and so on. Basically, you're rolling two dice and choosing the better/worse one before applying modifiers.