The following passage is the only one in Caesar's book, Commentarii de Bello Gallico, that mentions Titus Pullo and Lucius Vorenus. I've noticed that some other translations of Caesar's work gives Pullo's name as "Pulfio," but the S.A. Handford translation calls him "Pullo."
In the legion were two very brave centurions named Titus Pullo and Lucius Vorenus, both of them nearly qualified for the first grade. They were always disputing which was the better soldier, and every year the competition for promotion set them quarreling. When the fighting at the entrenchment was at its height, Pullo cried: "Why hesitate, Vorenus? What better opportunity do you want to prove your courage? Today shall decide between us." With these words he advanced outside the fortification and rushed into the thickest place he could see in the enemy's line. This brought Vorenus too over the rampart, hastening after his rival for fear of what everyone would think if he lagged behind.
Pullo stopped a short way from the Gauls, hurled his spear and transfixed one of them who was running forward from the ranks. The man fainted from the wound, and his comrades covered him with their shields, at the same time showering missiles upon Pullo and preventing him from advancing further. His shield was pierced by a javelin, which stuck in his sword-belt; and as the blow knocked his scabbard out of place, he could not get his hand quickly to his sword when he tried to draw it, and was surrounded by the enemy while unable to defend himself.
His rival Vorenus ran up to rescue him in his distress, and all the Gauls immediately left Pullo, who they thought had been mortally wounded by the javelin, and turned upon Vorenus. Vorenus drew his sword and fighting hand to hand killed one of his assailants and drove the rest back a little; but pressing on too eagerly he stumbled down a steep slope and fell. It was now his turn to be surrounded, but Pullo came to his aid; both of them escaped unhurt and after killing a number of the enemy returned to camp covered with glory. Thus Fortune played with them in their struggle for pre-eminence: bitter rivals though they were, each helped and saved the other, so that it could not be decided which was the more deserving of the prize of valor.
-- Gaius Julius Caesar, The Conquest of Gaul, Book V.44 (S.A. Handford translation; Penguin Classics, p. 125)