Reloading a crossbow and firing rapidly makes way more sense than firing and reloading two hand crossbows.

I do traditional archery. Crossbows are not my thing, but I've played with both regular and hand crossbows.
Getting off more than one bolt in 6 seconds means you are damned good, but I have seen it happen. (well, I saw 9 in 30 seconds, which is close) Firing 3 times in 6 seconds would be superhumanly fast.

Using a crossbow in each hand doesn't work at all without some sort of magical (or maybe gnomish engineering) explanation.
Reloading a crossbow (hand-crossbow or otherwise) is simply a two handed operation.

That said, this comes down to straight style, since the mechanics work out the same. In things like that, I usually default to letting the player describe it the way they want.