And:

http://www.lesmcleanphotography.com/...ull&article=21

There are those that will argue that there is nothing you can do with split grade that can't be done with a single exposure at a single grade filtration. And, for the most part, that's true. However, it is a valuable technique that many fine printers (such as McLean) have made use of. I use it when a negative is just a puzzle to me. Fortunately, most of my negatives aren't puzzling. Still, I appreciate having been taught this method by Les some years ago.

It just so happens that I returned only an hour ago from a local theater supply house where I bought a blue and a green Lee gel to use for this very purpose. I've always used the yellow and magenta filters, but many use blue and green filters. I'll do this with a condenser head and see how it goes.