Twinsets give you more gas. This means that in case of "issues" -- there's lots to choose from -- then you've more than enough gas to get to the surface.
A tiny pony -- 3 litres -- is too small for a slow, safe, low-stress ascent in an OMG moment. Will a 40 have enough gas for your leisurely ascent?
ali40 = 5 litres at 200 bar = 1000 litres. Previously we'd calculated that you'd need pretty much all of that for an ascent.
2 x 12 litre twinset at 220bar = 24 x 220 = 5280 litres. In other words loads of spare gas. Plenty even to help others during their OMG moments.
Same for sidemount where there's two completely redundant systems; you breathe them down and periodically swap sides.
