For me, it greatly depends on the individual horse and the situation. On my older, very broke horses, I only ever ride with 1 hand unless I am specifically working on something where I need more lateral signal.
On my younger horses, when they neck rein, I'll start riding them with only one hand. If a stiuation comes up where I need 2 hands (like working cattle where I need quicker reactions and more precision), I'll often go back to using 2 hands. But, as soon as the work is done, it's back down to 1 hand I do.
Once a horse knows how to neck rein, I try not to ever ride them with 2 hands during just regular riding. Riding them like that all the time makes it take longer for them to get solid and consistent with their neck reining.