Hi Jarrado, nice to meet you.
As far as I know, the reloading time of a unit's attacks determines how much time passes between multiple projectiles fired, so, setting reload time to 0, will do the trick, but the unit animation will be tricky, indeed, the projectiles will be shoot before the attacking animation starts, that's a bit ugly i admit, but this only happen to "living units" class, such as archers, infantry and cavalry.
Instead buildings, ships and siege units class can have a simultaneous projectiles spawn even if owning a proper reload time higher than 0.
Hope it helps in some way, but for more clearments, I suggest you to ask
here.
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MY GAME VERSION:
Age of Empires II The Conquerors 1.0 without any patches, just the ES release in August 24th 2000, proud to playing this game version!