The Team Dimension Data rider, 36, won with a solo attack in the final 10km of Sunday's road race on the Isle of Man to emulate David Millar's 2007 double.
Chris Lawless was second and Ian Bibby third, while Mark Cavendish, racing for the second time after three months out with glandular fever, finished 21st.
In the women's race, ex-world champion Lizzie Deignan won her fourth title.
The Boels-Dolmans rider, who won the Tour de Yorkshire in April, caught long-time leader Elinor Barker in the final stages of the 103.7km course.
Katie Archibald and defending champion Hannah Barnes were second and third respectively.
"It's one of the hardest national championships I've ever won," said 28-year-old Deignan.
"I'm very proud of the fact that we have so many women now who are capable of competing internationally as well as domestically."
Cummings, who won the time trial title on Thursday, went clear in the final stages of the 193.7km men's road race to leave Axeon-Hagens Berman rider Lawless and Bibby of JLT Condor trailing.
2014 and 2015 road race champion Peter Kennaugh, racing for Team Sky in front of a home crowd alongside fellow Manxman Cavendish, was fourth.
Thursday was Cummings' first race since he fractured his collarbone, shoulder blade and sternum in a crash at the Tour of the Basque Country in April.
"It was a really tough race. That style of competition is what you miss when you haven't been racing, it was punchy and explosive," he said.
"It's nice to be back doing what I do and I'm looking forward to whatever is next. It'll be nice to be at the Tour de France and to get stuck in."
Team Dimension Data, for whom Cavendish also rides, are yet to announce their squad for this year's Tour, which begins on Saturday.
Cavendish, 32, has the second highest number of stage wins in the race's history with 30 - four fewer than legendary Belgian Eddy Merckx - but he is not certain of a place in the team following his recent illness.
Steve Cummings became the first man in 10 years to win the British national road race and time trial titles.