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The pair gained a lap on the field in a thrilling madison race as New Zealand claimed silver and Germany took bronze.
Britain's Elinor Barker won bronze in the women's points race, but Jason Kenny failed to place in the keirin.
Great Britain's medal haul stands at three gold and one bronze, with Olympic champion Laura Trott, 22, leading the omnium heading into the final day.
Trott made an excellent start, winning two of the first three races in the six-discipline event - but she was pushed all the way by Dutchwoman Kirsten Wild, who is just two points behind the Briton.
The omnium resumes on Sunday with the 500m time trial, the flying lap and the 25km points race still to go.
Doull, 21, and Christian, 24, who won team pursuit gold on Friday, had to dig deep to prevent Australia taking a lap in the final moments of the 120-lap madison and deny them victory.
"The crowd lifts you so much," said Doull. "Usually when the lactic acid kicks in and you're really suffering, the noise carries you. It's unbelievable."
Barker, 20, who claimed gold in the women's team pursuit on the opening day, attempted to snatch the points required to repeat her silver at the Commonwealth Games with a late attack in the 80-lap points race, but was caught with a lap to go.
Great Britain's Owain Doull and Mark Christian won gold on day two of the UCI Track Cycling World Cup in London.