A few random observations about doubles:
--Allows higher total load.
Double cleans and front squats are exercises that feel great (to me) and provide greater overload than a single bell can. After doing a lot of double cleans, my single bell swings and snatches fly up. IMO, these are the killer apps of double bells. Some would say double presses or C&J, but I think overhead exercises with doubles are not as accessible for a lot of people because...
--Double overhead work has a much higher mobility demand.
A single bell gives you a lot more wiggle room (figuratively and literally) to wedge under the weight and support it overhead.
Double bells leave you much more "locked in" in initiating a press. You can't "push yourself away from the weight" the same way you can with one bell.
A lot of people can not easily support locked out doubles overhead mainly with alignment and structure, and have to use a lot of muscle power to fight their way into, and hold, the lockout. A lot of people also can't lock out doubles without hyperextending the lumbar spine.