???The only real difference is twice the weight and gas. If your dive plan uses a lot of gas, there's little reason one cannot use doubles.
My single tank is a 15 liters Faber at 232 bars
My twin tanks are Technisub Aralu, 9+9 liters at 200 bars. Both tanks contain the same amount of gas (roughly 3600 liters) and weight almost the same.
The twin tank is more stable on your back and the trim is easier.
Twin tanks do not need to be very large, they are often small and compact..Here in Europe for rec diving currently the preferred size is 7+7 liters...
Only problem is that they are more expensive.
When I needed more air, I also used a pony: it is perfectly compatible with twin tanks, you attach it easily in the center of them.