Do you mean that the ART300 is developed in a warmtone developer and then you are using the selenium to darken and cool the shadows? In that case use a weaker dilution of the toner (or a cooler temperature if you are using it hot) to make the change a little slower and easier to control. It is also an excellent idea to make a test print of the critical area of your production print, then keep that test (wet) next to the version being toned - it makes it easier to see small changes by direct comparison.
There are ways of split-toning in both sepia and selenium, but someone who has tried that will have to comment. You could try "split-toning" in the site search function too.