I guess the player wanted the game-mechanic rate of fire, not the image of a two-gun crossbowman?
He wanted the two-gun image, and he wants to replicate Western tropes in D&D, eg the stranger striding into a bar, playing poker, wearing a duster jacket and a white hat, etc. I pointed out that the repeater crossbow is much like a pump-action tube-fed shotgun, or a bit like a lever-action repeating rifle. He's rolling with the compromise. I think he also prefers a vaguely-plausible weapon, rather than "each hand crossbow re-tensions the bowstring, and sets another bolt in the groove, by THE POWER OF FEAT".
He asked about fire-arms, and I pointed out that muzzle-loaders are, if anything, slower to reload. (They also would not be crossbows, FWIW.)