Split printing for me is the ability to separate the negative , by using two or more filters and creatively using these separate filters to balance a print.
With graded papers one is locked into specific grades *exception being soft and hard double development* . Sometimes this locking in has its drawbacks , specifically one using a higher grade paper and burning in highlights. This for me was always problematic, as though the mid tones and shadows looked great, the highlights would look unnatural.
With split printing I would now print on a softer grade to hold beautiful highlight detail and I would control the shadows and midtones with a secondary exposure with a high grade filter to give rich blacks and contrast.
During the main exposure for highlights I would dodge the midtones and shadows, so that when the second exposure is given the higher filter creates the overall print contrast that I am trying to get.
You do not have to worry about the highlights during this second exposure as the work is all being done in the midtones and shadows and the density is not reaching up into the highligts.
Sounds complicated but really it is a piece of cake with practice.
For practice sakes heres what I did to learn this split printing method.
take 3 or 4 of your favourite images.
print each one on MC paper using only one filter to see what each filter is doing.
you will find out how long the shadow , mid tone and highlights, take to emerge.
This will also show you basic printing times for each stage.
Then start merging these tests by combining two exposures with different filters at the times you have already established.
What I think you will find is that when you merge the two or three filters the accumulated effect will make too dark of a print.
Basically back off your initial times for proper print density.
Now try making a very hard print with details in the highlights.
Now try making a very soft print with a hint of pure black.
Now try a ridiculas difficult negative and try to put good tone in each region.
Les Mclean and others will tell you how to flash the paper to make the most difficult image have good detail in highlights.
Its all a matter of practice and trying to visulize how each region*shadow,midtone,highlight* reacts to different filters.