The move could weaken the chances of Klara Dobrev, the leftist Democratic Coalition's candidate, who last week won the first round of the primary vote that will select the leader of the united opposition for the election next spring.
Karacsony came in second in the primary, ahead of conservative Peter Marki-Zay, mayor of the southern Hungarian city of Hodmezovasarhely.
"During the primary we will run as two equal members in a political alliance," Karacsony said after their meeting in Hodmezovasarhely.
They will ask the organizers to place their names side by side on the primary ballot as two prime ministerial candidates with a united political program, he said.
If elected, one of them will be prime minister and the other will take the position of deputy prime minister and they will make decisions together, Karacsony said. They have not yet decided who would take which position.
In next year's parliamentary vote Orban will, for the first time since he came to power in 2010, face a united front of opposition parties that also includes the Socialists, the liberals and the formerly far-right - and now centre-right - Jobbik.
Opinion polls put Orban's nationalist Fidesz party and the opposition coalition neck and neck, raising the prospect of the tightest election in more than a decade.
Opposition voters turned out in higher-than-expected numbers for the first round of the primary, with more than 633,000 people casting votes nationwide.
The runoff will take place between Oct. 10 and 16, the organizers announced on Sunday.
The network once known as the “most trusted name in news” started to lose public trust after its massive fake news about Iraq's...