In the case of a situation like that, I'd probably be inclined to roll them both separately and apply the higher damage result.Well the errata clarify that when two or more same-named spell overlap, only the effects of one of them—the most potent one—apply while the durations of the effects overlap. In the case of 2d6 fire damage (save for half) only one of them means that it doesn't really matter if you use the first or second casting, only one spell will have an effect.