1. The effect of adding small amounts of Dektol to Selectol-Soft to reach the necessary contrast is a bit different than using Selectol-Soft / dilute Dektol as consectutive baths. Beers A and Beers B work the same, and are virtually identical.
2. The advantage of using a single bath is when making an edition of prints, it is more consistent than consecutive baths.
3. "Consecutive baths" are an easy way to work for normal, low volume printing.
4. I expose the print to get correct whites in a normal time in the Soft developer, adjusting the paper grade, or filtration, as needed. When the scale of the negative is so long that my blacks are too soft ( after drying the print ! ) I use the Hard bath as needed.
5. it is important to dilute the Hard bath. Dilution does not change the contrast, it merely slows the development rate. ALSO, adding Pot Bromide 10% will effectively shield the whites from the effect of the Hard developer. ( note: mixing 120 from scratch, do not add bromide. Only add drops of 10% if you need it, which you probably won't )
6. Published formulae for both Soft ( Selectol Soft, Ansco 120 ) and Hard ( D72 ) are virtually identical when corrected for dilution.
7. The longer the print is in the Soft developer, the cooler the tone.
8. Using Selectol Soft / 120, one can expose to a higher density negative that will print with greater clarity than is common with a Hard developer. This alone solves the old problem of making different negatives for Silver and Platinum.
9. Higher density negatives also make it possible to make strong and clean shadows with low contrast filtration, reducing the need for N- development.
10. Glycin developers work very well with two bath technique. The Ansel Adams variation of Ansco 130 was a common application of the pre WW2 era. Without the HQ, 130 becomes an extra long scale developer capable of strong blacks without losing the whites of Selectol-Soft / 120.
11. Selectol is NOT the same as Selectol-Soft.
12. The effect of using Selectol Soft is absolutely NOT the same as changing filtration: the two techniques are HIGHLY complimentary.
Brother McLean does this technique
BACKWARDS 
with wonderful results.