Watch Live at Pompeii. They show clips of Floyd tracking vox for Dark Side of the Moon and in particular several of Gilmour and Wright doing their double-tracking brilliance. That's their biggest secret weapon, as Gilmour and Wright had amazing voices that double-tracked together like magic. I don't know if they're both singing on Comfortably Numb or it's just Gilmour double-tracking...but the answer to this question has been provided above several times: double-track, sweeten with verb. The rest is just in the quality of the voices.