It doesn't really matter if they "both signed off" on it. Tribe's connection to Obama is biased. Unlike Tribe, Olson had no similar connection to McCain:
Republican presidential hopeful John McCain said Thursday the question of whether he can run for president, despite being born in the Panama Canal Zone, was put to rest 44 years ago in Barry Goldwater's campaign. McCain added that he doesn't know why his campaign sought legal analysis from former Solicitor General Ted Olson of whether his birth outside the continental United States might disqualify him from the presidency.
link