Honestly, the only thing unrealistic about dual wielding crossbows is resetting with only one hand. I'm sure they could cook up some Dwemer magical mechanical engineering contraption that would make crossbows semiautomatic allowing for dual wield, but will they? Not sure. Besides, even if we find a way to make it realistic, I'm not sure if it would be practical..
Crossbows probably weigh too much to aim half-well in one hand as well. Although if they're a clever Dwemer invention for reloadings sake, they might also be light enough to get round that. I wouldn't completely hate it, but it's basically just a novelty that could turn out cool.
For bows, again I don't hate the idea of firing two arrows, but on this count I don't think it should have anything to do with dual-wielding - neither equipping two bows nor two quivers of arrows. It should just be a perk you can unlock with marksman, if it's included at all - which again makes it a potentially cool novelty feature.