The dividing line is when the hard and soft exposures are roughly equal when using Ilford 00 & 5 filters.
If you are printing soft negatives you may want to use the hard filtration as the 'exposure control' first channel and the soft filtration as the 'contrast control' second channel. This becomes more important with softer and softer negatives.
The order, of course, makes no difference to the final print, it's only a matter of more intuitive control over split grade printing.
Most printers find this much too confusing and do all prints soft/hard and just muddle on through with the occasional very soft negative.
See 'Way Beyond Monochrome' for more information.