Gary Anderson, 25, and Sean Anderson 24, had denied murdering Karol Kelly.
Another man, Michael Dunlop, 21, was found not guilty of murder but guilty of attempting to inflict grievous bodily harm.
A judge told the Anderson brothers the only sentence he could impose was one of life imprisonment.
At Derry Crown Court, sitting in Coleraine, Judge Philip Babington said he would set a tariff to determine how long they will serve on September 9.
Mr Kelly died in Grafton Street in Derry in the early hours of March 4, 2018 after an altercation during which he was stabbed several times.Tags: