So the tried and true method of resolving singles in the double line is how they do it at Castlerock at Sugarbush. Someone will ski up and yell "any Singles" if there are then they pair up. If not then they get at the end of the line and wait for the next single to ski up. You very rarely see one person on the Castlerock Double chair.
Yup...that's the same way the tiny hill in the Poconos where I learned to ski did things too. A single line for a double chair is just a bizarre concept to me.