There are many ways to exploit halo's somewhat broken melee system, but people are still finding out the truest, most reliable, and easiest way to double melee.

The combo is B,RB,Y,B The RB is also a combo for the BxB like B,RB,RB,B it is a glitch that you cannot have full ammo to use (hence the first shot he took that missed) I think the way you did it has a higher chance of working each time.

It looked to me like you just do BYYYB. idk