you are correct about selenium working from the shadows up and gold working from the hightlights down. That is why some of use both. I haven't really paid that much attention to the specific amount of change, rather than there is a different look that i like very much.
I will be in the lab tomorrow and will check some prints more carefully with regard to the highlight density. Or perhaps we will be lucky here and Les will chime in with his usual great tips. He does a lot of duel toning with these metals.