You can do sepia then selenium, or selenium then sepia. The advantage of sepia first as has been said is you can see the progress as you go. Dilute your bleach up to 10% full strength and pull the print when you see the first signs of highlights bleaching back. After sepia go with selenium for a short time at a rather strong strength, 1:3 to 1:10. The problem I see sometimes is that once in the selenium the sepia toned print moves very quickly to a very very warm brown. If you're going to keep things subtle your times need to be short, no more than a minute if you want to retain any sense of a split tone.
I've been experimenting more doing selenium first then sepia. The advantages of this is that the print doesn't run away on you going too warm. Selenium toning for 1-2 minutes at 1:10, then make sure you wash completely! Then sepia tone as normal. i find this produces a more true split tone, but much more subtle and less warm than sepia then selenium. The fun is in experimenting. So get to it!