how do you handle it - do you subtract extra difficulties, that apply to only one of the two, from the result ?
reason and example: a level 15 adept uses scrying on a level 5 one, his poor younger brother
comparing his scrying to the second sight of his brother, he has an advantage of 10, so he should go unnoticed despite the +5 difficulty for very familiar.
now his couriousity lets him spy later on his brother's new girl friend (a level 5 adept, too), which he has only seen every now and then. but the difficulty is +15 for somewhat familiar, compared to his advantage of only 10.
so now the question - should he spy equally easily unnoticed, even though the much higher difficulty - or should the girl friend have a much greater chance of detecting such attempt ?