In this video I have 2 hydras attacking 2 different turrets. The top hydra clearly fires before the bottom hydra, yet the turret dies at the same time...so the bottom hydra somehow was able to move while also reducing fire-rate gap. Both hydras also took 20 shots to kill the turret so there weren't any unlucky 1/256 air shots (I just learned about the existence of these air shots last night when Artosis took 4 vulture shots to kill a probe).
I'm not sure if this has any practical use but I found it interesting. Maybe hydras running from zealots it could be utilized to run, fire twice, run, repeat? I dunno.
*also, i'm sorry for the quality. I don't upload many videos and i'm computer illiterate. I don't know why the quality sucks so much but you can still see what's going on.
If anyone better understands the why what and how of the mechanics behind the 2nd shot of hydras coming out quickly I would be very interested.