Accuracy... actually, that might work out. Maybe something called "Scope <X>" that automatically grants a bonus to attack checks?
This makes crossbows a good weapon to use Bullseye with. And improves their range since extra accuracy cuts down on the range penalty.
As for dual-wielding crossbows being unrealistic, let's not forget that this is the same system that allows a player to wield a Tower Shield and a Zweihander at the same time (Spear Basics), can cause death via simple insults (Aggro BMS and the Threaten action in general), and many other things. Dual-crossbows is pretty much under the same umbrella of "well a suitably heroic and badass character could do it, good thing this system assumes all PCs are badass in some way!"
Then make that a rule: crossbows can make use of feats like Two-Weapon Fighting or something.