I think when we look at everything holistically, we have to figure out where the true risk lies on every dive.
On some dives, where a diver may opt for one or two gas switches, carrying two bottles may significantly increase the amount of work a diver must do at depth. Ex. swimming into strong current. Could this harder work lead to a greater chance of bubble formation? If so, then do we lose some benefit of a two gas switch. If so, we have to ask ourselves, "If we ditch one bottle, which one?" Am I more at risk during the deeper portion of the dive? Would I be safer if I figured minimum gas from max depth to the Nx50 switch? Or, is my ascent route so easy that I have no doubt sharing gas to the 20 foot stop would be a breeze and I'm more worried about DCS efficiency?
Two good examples: The wreck of the JB King in Canada is in warm green water with lower visibility in the summer months. It's adrenaline junkie sideways skydiving in a current on steroids. Getting off the bottom safely with updrafts and downdrafts and twisted wreckage places the greater risk in an OOG scenario. I don't want to share gas to 20 feet, It's a long way in the crazy conditions. Meanwhile, 40 miles west in Lake Ontario, the cold water at depth may want you padding your risk in favor of reduced blood flow due to cold water in the 40 F range. Most wreck dives are down a line and up a line so ascent while sharing should be easy. Oxygen might be a better choice die to increased DCS risk.
I plan dives from practical experience.