Having used the combination I suggested for at least 20 years I can speak from a well tested personal experience. I often dilute my hard (D72 or Dektol) developer 1 to 3, the recommended dilution is 1 to 9 so it is a quite strong developer. The Selectol Soft is diluted 1 to 4 or 5 sometimes even 1 to 6, the suggested dilution is 1 to 2 making my dilution very weak. I place the print in the hard developer first and as soon as the first tone appears on the paper (usually 15 to 25seconds) I remove the print from the dev, drain it and place it in the selectol Soft where I leave it a give continuous agitation until the image reaches the desired tonality. Those on APUG who have seen me work this way will, I'm sure agree that the blacks are deep and rich and the highlights are very subtley on the paper.
Sorry to disagree with you here Ian but I have noticed no significant change in the Selectol Soft development as a result of carryover of hard developer. I have to admit that when I first started working this way I did consider the carryover may be a problem so I tested it and found no significant differences when making the same print at the beginning and the end of a printing session.
Obviously, as I said in my first post this is not the only way to work with two bath development and I'dstrongly suggest that experimentation is the best way to go to decide what best suits your taste.
For those who may be interested this is one of the demonstrations I'll be doing in my Darkroom Workshop at Foto3.