Do a search through the archives. There's been a couple of threads fairly recently that gave me my introduction to split printing. My understanding is that it gives you a broader tonality scale than single filter printing. Now, I only use single filter printing for very flat negs, and I working my film process now so I expose and develop to take advantage of split filter printing. If you can't find 2-3 monumentally helpful threads on the topic, PM me and I'll come up with some links for you.