I think that the bag that is not worn on back should have it's weight reduction lowered. You are more likely to easily carry the same amount of weight on back, than in hand. It can be a static value taken off it across the board or a percentage of the reduction. In case of BHB 80% that can be lowered by 20 or 25% (60% in each case when carried in primary/second slots). In case of SB 60% that would be 40% or 45% rendering it around the same as plastic bag, but with higher capacity. It could potentially be changed the other way around - any baggage that can be carried on back has it's weight reduction increased. That would require modifying the base value on each of them.