I also like it. I loved the Double Crossbow in the Waylander books.
Now how about a Crossbow/Matchlock combo? They did actually have them and it would be very Dwarven. There are a number of real world firearm/melee weapon combos also, axes, maces, warhammers, swords and daggers.
I concur. I don't think it would be unbalancing to leave the crit the same as a regular crossbow. I'd make reloading a move equivalent action to load one bolt or a full round to load both.