Bows - A basic guide to different types

[Skvez]

Any range weapon is better than no range weapon but with a longbow I can fire a 'warning shot' and get another arrow re-nooked before the other party closes the gap to Melee range. With a crossbow I (probably) can't.
Anyone approaching is likely to be creeping up carefully. Firing a warning shot is likely to be necessary. Or if they're creeping up with partial cover knowing when to fire your one-shot crossbow will be a terrible decision. Knowing you can 'waste' an arrow from a longbow and still 're-load' in time to get another shot off if they charge will be a great comfort (and tactical advantage).
There is a reason why double barrel shotguns have two barrels.
How common are twin crossbows? and are they really a practical weapon or a gimmick?