Hard drums give a smaller contact area so the grains on the abrasive belt penetrates further into the surface. Softer drums create longer contact area so the penetration is less. The longer the scratch the less depth of scratch.
I'm not sure who built your sander with the configuration you mentioned so that might help me with recommendations.
I would pick 80 and 40 shore for the two drums. Hard to cut and soft before going into the platen.
Does your machine have air exclusion on each head? How much actual stock do you need to remove?
We might be able to simplify things for you and eliminate two of your six passes with a scratch pattern that will melt away like butter.
When using a two head with three separate grit sequences you will never have a proper set up on all three. Only one sequence will be really set up correctly.
Also, be very, very careful of the book on sanding. It give information that confuses people and does not always help. If it gives correct information, but lacks the ability to convey the real world applications of that information, then it can become a huge hinderance.
Feel free to contact me if you would like to discuss this further.