Really, you should not be able to use two hand crossbows since you need to load your crossbow. This should require the crossbow master feat, which is not yet in 5e, but then, you should be able to add your dex to all attacks. I think when this is in the game they will stop letting you dual wield hand crossbows normally.
If you want to be a bit more drastic, you could also limit off hand bonus action attacks to once per turn, though I think rogues long term are kind of bad, so I don't mind them getting this little boost for more damage
