With split printing all you need are two filters. You just buy optical (taking-quality) glass filters, one deep green, the other deep blue, which
can be used below the lens. Life is simple. Even simpler is to use a conventional colorhead which can have the filtration reset in the dark. Then
you can do either true split printing or ordinary variable contrast work. I gave up even bothering with so-called grade numbers long ago. Split
printing is far easier to do than to explain. But not all VC papers are the same. With some you need at least a token amount of exposure of both
emulsions in order to obtain a good DMax.