Ok, nothing from the RAW, but simply thoughts of what would be logic: the main hand is normally the one doing the complicated tasks becouse it´s getting the main attention from the brain. So it would be logic to say there´s one main hand and all others are off hands. If you went for the "each pair" thing, the main hand of each pair would be at the same side, as main and off hand are partly defined by the dominant half of the brain. This way oneambidextrous quality would be enough for all pairs, but that is explicitely denied. So it seems to me that the "one main hand - all other off hands" would be more fitting.
I personally would go for the 5x ambidextrous quality, but depending on the char (and the player, if i know him) would give RP-boni on the BP, so i would not have to deal with a six-handed killing machine just becouse i allowed a flair thing.