If two Venerables were to enter the Heavenly Tribulation Realm at the same time, they could only kneel and die. One world, one kneeling technique.
In short, two Venerables transcending the tribulation together = seeking death!
According to Sage Monarch Winter Melon's knowledge, there had never been anyone from the cultivation or holy knight system that was so set on seeking death. After all, practitioners who were able to reach the Venerable Realm were usually big shots and wouldn't play with their lives.
Actually, in the ancient witch system, it was indeed possible to find 'two Venerables transcending the tribulation together', but this referred to a Venerable-level witch and their Venerable-level familiar transcending together. Such a situation was somewhat different from two non-related Venerables transcending the tribulation together.