I use an Opemus enlarger with a Meopta Meograde head vor BW-printing. This head contains yellow and magenta filters that are moved in variable measure into the lightbeam that goes into the mixing chamber. E.G. the yellow filter could be 75 % in and the magenta filter 25% for a given grade. The filters are mechanically coupled to keep the exposure constant.
As I see it, this is equivalent to split grade printing. The only difference is that in split grade printing with single filters the time per filter, not the intensity of the coloured light would be varied. In the example above, this would boil down to t seconds for the magenta filter and 3t seconds for the yellow one.
Do I see this right?