The 17-year-old won Wednesday's 50m butterfly final in a new European record and followed that yesterday with a 12-second victory in the 200m individual medley in a personal best 3mins 06.78secs.
Murdoch became the third-fastest man ever in the 200m breaststroke at the Games in Glasgow when he stunned compatriot and Olympic silver medallist Michael Jamieson to pick up gold on the opening night.
"Ross Murdoch's win got me fired up to race," said Mullen. "To go from no gold medals to two in 24 hours is quite amazing. Training has been going well, so I was hoping to do well here."
Ellie Simmonds set a new world record as she stormed to her third medal of the championships. Simmonds topped the podium in the final of the women's SM6 200m individual medley with a time of 3:04.07, almost five seconds faster than second place Yelyzaveta Mereshko of the Ukraine. Her heroics were preceded by Sascha Kindred claiming gold in the men's final of the same event, setting a new world record of 2:40.82.