In theory a single exposure would appear to be the same as two exposures using soft and hard filtration but in practice there is a significant difference, IMO. Giving two separate exposures allows you to dodge during one or both thus giving better local contrast control. For example, when the negative has "thin" areas the best way to introduce contrast and good separarion is to use harder paper grades. Split grade printing allows you to dodge out all or part of the soft exposure without affecting the remainder of the print. You cannot do this with a single filtration exposure. The same principle applies to burning in.
Originally Posted by L Gebhardt
I have taught split grade printing in colleges and at workshops for many years and have found that students can generally make good prints quicker and with less wastage of materials once they learn to control the method.