When Zhang Heng charged at Nasika, his partner did not help him fight off Zhang Heng.
The reason why he made such a decision was that he was too frightened. After a while, he calmed himself down and started to assess the situation. In his opinion, if Zhang Heng had concealed his strength earlier, he should not be much stronger than Nasika. In other words, even if Nasika could not defeat him, he should at least be able to hang in there for a while.
Thus, he chose this opportunity to kill Varo, the weakest gladiator in the arena. After that, he planned to work together with Nasika to attack Zhang Heng.
There shouldn't be any problem with this strategy. It was true that Varo was weak as he thought he was. As soon as he approached him, Varo started to lose his calm. Besides, retiarius was a profession that focused more on offense instead of defense. Once Varo lost his calm and began to focus on defense, it was equivalent to revealing his opponents' weakness.
