This story comes from one of the epigrams that the Roman poet Martial wrote about the opening of the Flavian Amphitheater (The Colosseum).
Here's the full poem (pulled from Ancient-orgins.net)
“As Priscus and Verus each drew out the contest and the struggle between the pair long stood equal,
shouts loud and often sought discharge for the combatants. But Titus obeyed his own law (the law was that the bout go on without shield until a finger be raised). What he could do, he did, often giving dishes and presents. But an end to the even strife was found: equal they fought, equal they yielded. To both Titus sent wooden swords and to both palms. Thus valor and skill had their reward. This has happened under no prince but you, Titus: two fought and both won.”
-Martial, ‘Liber spectaculorum’
Not much else is known about Verus or Priscus, nor what happened to them after they earned their freedom.
LINKS
Further reading
More from Martial's Liber Sectaculorum
Interesting discussion on gladiatorial combat in roman art and a breakdown of Martial's poem