I'd imagine Hydra being just oversized German 20mm Flakvierling 38.
Logically, it is better to have one weapon firing with a delay to lower the recoil.
Macharius's guns are both in one gun mount, but I'm sure they are both operated individually. Firing both at once will result in some serious recoil and possible damage.
But to summon this thread,
40k and logic are not the best friends and with the vast amount of artists, writers and such, consistency and, more importantly, knowledge of how physics and guns work
IRL is hard to keep it all together. So, if you see something like two guns firing both at once, just think that the author made it that way because it looks/sounds cooler, despite it being total
BS IRL.
The Imperium is not THAT backward to not to know how to shoot a gun so it won't damage itself or its mount/bearer. There are still some exceptions, though.