I use conduit a lot in Jamaica. It would save money, in the cost of the whole house install, to share the neutral. Yet I always pull two neutrals, two live and a ground... have often wondered if a second ground might be better but.. since I pull a full size ground, I dont think it really matters.
I always tape the two wires that are paired as well, to keep track of them. More common in my installs of the past five years in Jamaica has been the use of Red/Black for pair one, and Brown/Blue for pair two... since Jamaica uses the British old colors, and we are allowed to remark at the ends. Inspectors actually like my method as it is easier for them and I to keep track of circuits in the conduits, and follow them to the breakers, if the labels fall off.
But, very seldom do I use any wires in Cables, such as 12-2 or 12-3... and when I used to do that in the USA, I always ran it as 12-2.. because my boss hated running 12-3 and sharing neutrals.. insisted it was better to run the two cables from the panel, and that it cost him less money in the long run.
Only 14-3 we ran was for the light switches..lol